Is there a guide for replacing iPad Pro 12.9" battery?

Sorry no guide, but you really don’t need one! The teardown guide will give you enough info to figure things out iPad Pro 12.9" Teardown. Here’s the needed steps:

Step5 & 6 - Removing the display

Step9 - Remove plate, disconnect iSight camera from logic board.

Step10 - Remove shield

Step11 - Just disconnect ribbon cable from logic board

Step12 - Just disconnect speakers form logic board

Step14 - Disconnect antenna cables from logic board and carefully lift them past the edge of the batteries and the remove the two clips

Step15 - Disconnect the other two speakers and the last ribbon cable to the switches, then lift the logic board up and out.

Step18 - Carefully heat the backside of the case to soften the pull tapes adhesive using a hair dryer or heat gun on low. Using a pair of forceps like this one: Debakey Lung Forceps as they allow you to pull the tapes more effectively as you don’t want to tear them as you pull.