Actually, there is no way to do this within the software. But you can do it. You'll just have to modify the language catalog.
Edit the file <TD>\lang\en-us\messages.tournament.properties
where <TD> is where ever you have installed the Tournament Director, typically "C:\Program Files\The Tournament Director 2".
Search for "Tournament.AddOnRebuyInfo.LastRound", you should find:
Tournament.AddOnRebuyInfo.LastRound=Through Round {1}
Change the text and restart the software. Note that when you update the software, the language files will be re-installed so you will lose your changes. If you want to work around this, create your own language by copying the "en-us" folder and renaming it. You'll probably also want to edit the "description" file (in the copied folder) so you can distinguish "your" language from the default English language. Then change the language on the Preferences tab.