Phones, just like pretty much everything else have a useful life time after which they need to be replaced with new ones. This life time is determined by a mix of technological advancement and planned obsolescence by manufacturers. In the first case new apps come out and at a certain point the hardware is simply not powerful enough to run the apps anymore. In the second case, design choices and software updates by manufacturers limit how long a device can be used before it breaks or before it becomes too slow to run anything and there is unfortunately not much that we can do as users to avoid that.
A third factor affecting the life time of a phone, tablet or any other electronic device for that matter is accidents: phones fall, screens crack, batteries wear out, charging ports stop charging, water gets in, mics and speakers start having static... if you are reading this then you probably know far too well!
Most people will take their phone to a repair shop, some people will put it in a drawer and buy a new one, and a few will want to try and fix it themselves. If you are part of the last group because you want to save money or because you like a good tech repair, congratulations for taking the step! Tech repairs are fun and exciting (at least that is why I am in it).
It is definitely possible to fix some of the common issues yourself, here is what it takes.
Technical knowledge
It goes without saying that if you want to fix your phone, you need to know where to start and how to go about doing it. There are many resources available online for free. Quality videos on Youtube explain how to teardown a phone or how to fix a cracked screen in great detail among other things. Carefully following the step-by-step instructions should get you a descent result. Do keep in mind that some videos skip important steps so if you find yourself confused about how they went from one step to the next one, check a different video and you might find the answer.
Also, because most videos are made by experienced tech people, they tend to make some steps seem a lot easier than they actually are. For example, the glue holding an iPhone screen to the frame is a lot stronger than what it looks like in most videos which makes the most important initial task of opening the phone to access its internals pretty difficult for the unexperienced. Cracking the screen while trying to open a phone to change the battery would not be a great outcome! In general, if you need to use strength or force, you are probably doing something wrong. Go a few steps back and make sure that you followed everything properly.
Equipment and Parts
Having the right tools for the job makes all the difference in the world in how successful a phone repair is. Fixing a phone without special equipment is doable but is a lot harder and the risk of doing more damage is much higher even for an experienced technician. Some equipment, like a high quality heating mat, an anti-static work mat, and a set of precision screwdrivers and tweezers are essentials that you will use for many repairs. If you break your phone regularly and you would be doing multiple repairs it will make sense to invest in this equipment and learn how to use it. For those looking to fix a cracked screen or replace a battery once in a while it is probably cheaper to visit a reputable phone repair shop and let them do it for you than buying your own equipment.
You can find and order equipment and parts online. Always stick to OEM and reputable aftermarket brands for your parts to keep your phone in the best condition. Depending on the phone that you are trying to repair, some parts may be harder to source and require that you salvage them from other phones.
In the specific case of batteries, make sure to only purchase a genuine OEM battery or a high quality aftermarket one. There are many cheap batteries in the market but they can pose a significant risk of fire or explosion, so stick to reliable brands even if you need to pay a bit more.
Patience, will to learn, and risk tolerance
This is probably the most important element that you will need. Make sure to create a back up of your data before attempting a repair to avoid losing anything important in case you break the phone in the process of learning. The process can be extremely satisfying but there are also frustrating moments when things don't work as expected. Finding the issue and solving it makes the satisfaction from completing the repair even greater.
It is best to attempt your first phone repair on a device that you don't mind damaging further. This will reduce your cost of learning and allow you to practice more comfortably knowing that making a mistake will not cost you your main phone.
Finally, it is important to know the limits of your equipment and the available knowledge and keep in mind that you can always visit a reputable repair shop near you to have your phone fixed or the repair completed.
Have fun learning and fixing!