How do you replace a battery on a Galaxy A21

It does rather sound like it's time for a new battery, so I think you're on the right track.

I wasn't able to locate a battery replacement guide specifically for the A21, but they are available for the A21S. I can't tell you for sure how similar the two phones are, but from what I've seen in the past, a minor change in the model number like adding the "S" usually indicates something like a processor upgrade rather than a complete redesign, so I believe the A21S guide will most likely work fine for replacing the battery in your phone.

Manuales / Samsung Galaxy A21s / Battery

The site is in Spanish but has options for other languages; I generally don't have any trouble using it, especially with the Chrome browser that has translation available if needed.

Just use common sense; there may be a screw in a different spot or something, but in general the principals are going to be the same; you'll have to remove the rear cover, taking care if it has a fingerprint reader there, then take off the midframe over the motherboard so you can get to the battery and display connectors that will need to be unplugged. After that you'll want to carefully remove the battery; 90% or higher concentration isopropyl (rubbing) alcohol can be your friend here to loosen the adhesive if the battery proves to be stubborn. Above all don't use metal pry tools; trust me, I killed an iPhone X doing that, so don't be me.

Good luck, and let us know how it all turns out; your experience for better or worse could help the next guy who has to replace his battery.