I am wondering if it is possible to edit curves of action already put in the NLA editor. And suppose that I have an action that extents from the frame 0 to frame 40 and in the NLA editor Strip Extents is from 1000 to 2000 (I scale the action). However, I discover that the action is not very tuned and I need to move hands or rotate little bit the head. How can I edit this in original action not in strip. Hope that my question is clear, otherwise, I can put my file.blend
Position your cursor over the track in the NLA editor and hit TAB. This will enable the editing mode for that NLA track (it should turn green). If you have an F-curve editor open at the same time, the animation curve for the NLA track will appear in it a allow editing. (Ensure that the 'Only include channels relating to selected objects and data' option is NOT selected in the F-curve toolbar).