To trim a layer in the timeline, drag the triangles on either end of the clip to a new point in time.
If you are at a frame when you want an edit to begin, press Option+[ (that's left bracket) (Alt+[ ) to trim the beginning of the layer or Option+] (that's right bracket ) (Alt+] ) to trim the end of the layer.
Once you have in and out points, you can slip your edit in the timeline using the Pan Behind Tool.
Select the layer
Choose the Pan Behind Tool
Mouse over the layer in the timeline and drag either left or right to slip the edit.
To trim numerically , expand the In and Out Columns and enter your desired In- and Out-points. Changing Duration will stretch the layer.