How to modify selection using ALT/CTRL/SHIFT with arrow keys

I want to select part of a clip and loop it.
To do this, I need to fine tune edges of the selection.

I want to shorten the selection on the right side.
I tried using CTRL and ALT and SHIFT with the arrow keys, but it doesn’t work right.
I thought SHIFT and ← would work. It does not. It only expands the selection on the left side.

How do I make the selection shorter (move selection left) without de-selecting ???

If you only have a cursor position, but not a selection, pressing LEFT or RIGHT moves the cursor to left or right respectively, giving you a new cursor position from which to start a selection.
To create a selection starting from the cursor point, hold SHIFT while pressing LEFT or RIGHT arrow to extend the selection leftwards or rightwards respectively from the cursor point, or to further extend the selection.
If you extended the selection too far, hold CTRL + SHIFT while pressing LEFT or RIGHT arrow to contract the selection from the right or left edge respectively.