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there is adhesive under the battery that requires a fair bit of pressure to make the battery come out. Pulling on the corner of the battery pull tab is more effective than pulling halfway along and it did come out eventually. heating may help but heating a battery is never a good idea heat is going to take quite a while go get through from the screen side as there are many layers to get through before you get to the battery compartment. best bet make sure it is not cold then pull from the corner carefully
After this step depending on the spare part you are using, you may need to remove the metal back and the home button touch sensor cable and socket. this is the socket that the fingerprint reader attaches to, and that you unclipped from in step 21. The cable is stuck to the back of the metal screen and the socket is attached to the plastic base so be careful when removing the back that you don't rip the cable off. the socket is glued to the plastic and comes off with some careful spudging. I did not have to heat it but that would probably help too. once its off you can easily attach it to the new spare part. Hope that helps, cheers Brad