To use the scissors (U) to delete just part of a tie chain, invoke the note entry caret by pressing Return (or double-clicking); the caret will be at the first note of your tie chain. Use your arrow key to go forward through your tie chain until the cursor is just before the desired cut point; then type U.
(The cut point is at the end of the tie you wish to cut, not the beginning.)
In Engrave mode you can select just one note in a tie chain, but you can’t delete the tie in Engrave mode.