Changing Selection for the "Center Across Selection" alignment

I'm sure others must have had this problem. Hopefully someone can help me
fix it. When I format a cell as center across selection, it centers the info
over whatever area I had highlighted. But once I've done it once, I can't
figure out how to change the selection to center the text over.
For example, let's say I center A1 over A-H, then I delete the information
in column G and H and I only want to center the info over A-F. No matter
what I do I can't seem to change the selection.
I've tried highlighting the new area, and hitting "center across selection."
I've tried deleting the info and reentering it. I've tried left alighning
it, then centering it again. No matter what I've tried, I can't seem to
change the selection my text centers to. Thanks for the help.

You just highlight the G1:H1 range and give it any other alignment format. For example click on the left align icon, right align icon or centre icon.