Changing the battery is pretty straight forward. Unscrew the back and pop off the cover. The battery you need to replace will be right in front of you - impossible to miss! Just take it out and pop the new one in, replace the cover and put the screws in and away you go!
Remember the get your old tamagotchi back, you will need to choose "download" after you have finished.
This is a copy of an older version instruction sheet. Not much has really changed though so if you scroll down towards the bottom there is a diagram that describes how to change the battery if you aren't sure. https://www.mimitchi.com/tamaplus/manual.shtml