Take a look at Replace Field Contents.
This tool can modify data in large numbers of records and cannot be undone, so research it in help and make a back up copy before using it for the first time.
Thanks for the Replace Field Contents suggestion. That works great for when I want to change what's in an entire field. Any suggestion for how I could add text, not replace all the contents of a field?
For instance I have a database I've built to track my marketing efforts. Under the field for "Promos Sent" I'd like to just add text referring to the latest promo I sent leaving the previous text as well.
So if only 75% of my records have something like "Golfer" and I want to add "Boeing 787" to all the records without losing the "Golfer" text that is only in some of them how would I do that?
First, this sounds like you are packing data into a text field that would be better added to a new related record instead.
But if your field where named "TextField", the calculation you could use might be:
YourTable::TextField & "Boeing 787"
or you might use:
List ( YourTable::textField ; "Boeing 787" )
either of these expressions could be used inside a replace field contents calculation. The second option starts the appended text on the next line as it will insert a return between the existing text and the quoted text that is being added to the field.