Hi @keralite
I find the best way to do this is to apply a line of Isopropyl Alcohol 99%+ along one side edge and the bottom edge of the battery, using an 18G blunt tip dispensing syringe,¹ allow the alcohol a few minutes to soften the adhesive and then gently, use a pry tool to lift the battery up a small bit.
Then apply some more alcohol under the slightly raised edge of the battery, wait for the adhesive to soften a bit and then pry the battery up further and so on until it is completely removed from the case.
Notes:
¹ Examples only. You could always ask your local pharmacist if they have anything suitable that you could use. Just explain what you want to do with it ;-)
Do not attempt to remove the battery all at once. Raise it a bit at a time.
Do not use a metal pry tool as you may easily puncture the battery. Old credit cards or guitar picks can work if you don't have a plastic spudger.
Remember to disconnect the battery cable from the systemboard before commencing the removal procedure and to dispose of the old battery responsibly at a hazardous waste collection point in your location.