Obviously you know you'll need to replace the battery. Sorry to say that iFixit doesn't have a guide for your phone, but there's a Spanish site called Nadie Me Llama Gallina that has just what you need.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-a02s-1313/bater-a-11793|Manuals / Samsung Galaxy A02s / Battery|new_window=true]
You can look around for a replacement battery; most of the major sites like Amazon, eBay and AliExpress will probably carry it. Here's the first one that came up on my cursory search.
[link|https://www.amazon.com/Replacement-Battery-HQ-50S-Compatible-SM-A025F/dp/B096DG27RB|Amazon.com: E-YIIVIIL Replacement Battery HQ-50S Compatible with Samsung Galaxy A02S SM-A025F/DS with Tools : Cell Phones & Accessories|new_window=true]
That should do it; let us know how it goes!