My phone's battery

Ashenafi, your question isn't clear since you don't seem to have finished your sentence, but I'm guessing you want to know how to replace the battery.

As far as I can see, there aren't any guides available specifically for that repair, but there is a teardown video that shows you everything you need to know to remove and replace the battery.

Tecno Spark 8 Pro Disassembly Teardown How To Open - Tecno KG8 Disassembly Teardown Repair - YouTube

Follow the video until you've removed the cover over the motherboard, then stop. At that point all you need to do is to take off the fingerprint sensor to get it out of the way, then unplug the battery. There shouldn't be any need to take out the motherboard or do anything with the charging port board or its cover. Skip forward till you get to the battery removal part. As you'll see, all you really need to do at that point is to soften the adhesive with either heat or 90% or higher concentration isopropyl (rubbing) alcohol, then carefully pry the battery out of the case. I would strongly suggest not using metal tools, as they can puncture the battery covering, at which point you'll be needing a new battery. Try not to bend the battery either, as that can damage the internal structure. A plastic spudger tool or card should let you get it out without problems.

As you work, take lots of pictures and when you're done you can use them to create your own iFixit guide to that repair. It's easy, fun and will earn the appreciation of the repair community, especially those with the same problem.

Create a new Guide - iFixit

Good luck, and be sure to come back and let us know how it turns out or if you have any questions.