If it is just the text you want to update, there is no need to update the template, you can do it from your phrases
If you make template edits, the edited templates don't get automatically update when you upgrade (after an upgrade, you have to revert to allow them to update to the latest version, and then you have lost your customisations)
Instead, find the phrase "sign_up_now"
You can find that by
logging into the ACP >> Appearance >> Search Phrases
Then search for the phrase "sign_up_now"
Just about all of the XenForo template text is added here, and you can update them to better suit your forum