One way...

Assume your data is in the range A2:A10.

Enter this formula in B2 and copy down as needed:

=REPLACE(A2,1,2,"AC-")

Inspect the results and make sure it is what you wanted.

Select the formulas in B2:B10.
Right click>Copy
Right click>Paste Special>Values

You can delete the original data in column A if so desired (or, delete the entire column A).