You could try using Edit Program, which opens in a text editor, and replace 5.18 with 5.00. If there are no dimensions at 5.18 that you want to preserve, just to a mass replace.
In Creo2 it's:
Tools > Model Intent > Program > Edit Design
(Warning, if it's an assembly, you'll need Advanced Assembly to use this.)
In Reply to Wayne Falco:
Is it possible to add a temp relation that says IF a diameter = 5.18,
then make it = 5.00? Gotta lotta holes to change and thought maybe I
could change them in one fell swoop.