Animations : How to change time of frames

Hi!

How can I edit the time in between 2 frames of an animation?
The global parameter ‘playrate’ does it for the entire animation, i dont want that.
I have this animation and I want to slow down the buildup, keep at normal speed the release and hasten the recovery.
ie.:
Frame 30 : move from second 1 to second 2.
Frame 60 : remains the same
Frame 90 : move from second 3 to 2.5

Without opening Blender. Ideally from an AnimMontage.

Thanks <3