Oh, I've never made a mistake on a WPS before......Actually, you need to simply strike a line through the error and write in the correct info, (like I do and many others who make mistakes) initial, and date. At the top of the form, (at least on ours) there is a place to designate the revision and it's date. It would look nicer if you retyped the WPS and placed the revision number and date on it, and keep the old one with the typos in the file behind it for reference, probably not necessary to keep it, but...
John Wright