Let's say that you used your mouse to select a block of text, clicking and dragging from left to right. But you dragged a bit too far.
First, you need to know where your cursor is. If you dragged from left to right, the the cursor is at the end of the highlighted text. If you dragged from right to left, the cursor is at the beginning of the text.
If you're at the end of the highlighted text,
Use Shift-[left arrow] to back up character by character, unselecting text as you go.
Use Shift-Ctrl-[left arrow] to back up word by word.
If you're at the beginning, use shift or shift-ctrl with the right arrow.
15 ways to select text in a Word document