Oh thats a tricky one. Start with the one with the broken display to learn, since the display is in danger during this procedure and you need to learn how to do it right (battery is glued to the screen). Often a solvent is used, in addition I use a thin thread behind the battery to carefully losen it.
Have a look at the video I found to get an idea about the process.
https://www.youtube.com/watch?v=KtjaGMavR_A