@xavierjazz follow something like this video [link|https://www.youtube.com/watch?v=uRwAhsEYkMM|Samsung Galaxy A21s Battery Replacement A217 - YouTube|new_window=true]
Since there are no guides for iFixit yet, how about you create one for the rest of us? Just take lots of pictures and follow this guide [link|https://www.ifixit.com/Guide/new|Create a new Guide - iFixit|new_window=true] It would really help others out.