How do I fix power botton

Drew, is it the mechanical button - the part you put your finger on - or the electrical part that's not working?

In other words, is it physically damaged such that it's not functional, or is it intact but just doesn't do anything when you press it? Does it click when you press it?

Because those are two separate repairs, so it won't do us any good to explain one whole procedure if it's the other one that's the problem.

Once we know exactly what the problem is, we can talk in more detail, but in the meantime here's a guide to replacing the electrical switches for all of your buttons.

Manuales / Samsung Galaxy A21s / Side push buttons - Nadie Me Llama Gallina

@drew48405 Okay, maybe I'm slow, but I'm still not clear as to whether there's physical damage to the case and buttons, or if they just don't work. If it's the latter, you'll want to use the guide I posted to replace the flex cable with the electrical switches; otherwise my best guess is you'd need to replace what they call the "intermediate housing", which I believe is the part that holds the mechanical buttons. The button replacement guide shows how to remove it if you just follow it through Step 4.

It might help if you could add a picture of your buttons so we can see what you see.

Adding images to an existing question - iFixit Repair Guide