OK - the 'easy' way first. (FWIW, IMO there's no easy way to get a grip off that's been attached with solvent and 2 sided tape - let alone a putter grip (the hardest ones to save of all - my success rate is somewhere around 50%).
That said, use an air compressor (not 12v car tire type). Squirt grip solvent down the vent hole (be generous!). Put the protector tube over the grip, and add the air. Gently twist the grip to help it release.
If you don't have a compressor, the other method I'm aware of is to inject grip solvent under the grip (a syringe slipped up the mouth of the grip is the method). Then you use a 'grip saver' tool (I've seen wire coat hangers used), to work the grip free of the tape.
It's certainly not easy, in either case. (It's dead easy for a grip installed with air). By the same token, the grip is only a few bucks... and it sounds like you're flirting with the darkside.... so you might as well order grips for your
irons along with a replacement putter grip!