Well...
If by FIXED DATA you mean that the field ALWAYS shows "ABC-123" then why bother to store it at all.
But if by FIXED DATA you mean FIXED FORMAT of 3 alphabetics and 3 numerics - but ALL characters stored as the text representations thereof...
Look up the MID$ function or the LEFT$ and RIGHT$ functions as ways to pick apart a field. Then write an UPDATE query that picks the first four characters (xxx-) and the last three characters (nnn) and concatenates them around a literal 0 character.
Might be something like an Update for which the update to: element for this field would be LEFT$( ) & "0" & RIGHT$( )
I'll leave it to you to read up on those functions to figure out the right syntax.