you create a file named " Stringmods.xml "
And begin it like this
<stringmods>
<StringTable>
<Language name='English'>
<!-- change the text of the Continue button on the Main Menu -->
<String _locID='20038'>ModContinue</String>
</Language>
</StringTable>
</stringmods>
The LocID is the orginal string ID you want to replace.
After youa re done you place it in _>
Mods\local\YourModName\data\strings\english
Done you made your string mod.