You can mark an in and out on the clip in the viewer and an in and out in the timeline and edit: fit to fill.
You can select a clip in the browser or load the clip in the viewer and hit command-j
If you want to adjust the speed of a clip that’s already in the timeline
First step: To avoid adjusting the position of other clips in the timeline, move the clip to a new higher track or a track with no other clips after this and lock all other tracks. Option clicking on the lock symbol of this track will lock all other video tracks and shift F5 will lock all audio tracks.
If you want to change the speed to a constant amount, select the clip and hit command-j
If you want to make a variable speed adjustment: doubleclick on the clip to load it in the viewer and go to the motion tab and add keyframes in the speed area adjusting the speed at each keyframe. Note; if you have a multiclip, you need to match frame back to the original clip and replace the clip in the timeline. You can not make variable speed changes to a multiclip even if you callapse it.
Variable speed changes are extremely powerful but a little tricky. For example to synch a music performance visual to a different take of the same piece, I add keyframes on the beat to the clip and keyframes on the beat to the audio I’m trying to synch it to. I adjust the speed at at different places until the markers match.
IMPORTANT: to get the best quality speed change, after you’ve applied your speed changes and are happy with the result, send the clip to motion select the clip in the timeline and go to inspector: properties: timing and change the frame blending to optical flow. Save and return to final cut and render.