Well, there are language files in /public/language. The GDPR text is, as far as I remember, sourced from a few of these entries (the language files are JSON files with a prase identifier, and translation). So if you want to change the wording, you would have to modify these files in your clone of the repo.
Not sure if this is the "best way" since they might be overwritten when you pull the next version (could someone with more knowlede than me confirm this?) but that's a git issue that you should be able to handle once you get to updating.
For example, I see three entries in the file "/public/language/en-US/register.json" that start with "gdpr_". There are probably others but this is a good place to start. Just search those language files for the exact wording.
Also note that to chnag eit you would have to change all of the languages, but if english only is enough for you, that is easier.
MongoDB changes licensing creating cascade of open source requirements.
Take a look, if you are deploying NodeBB with MongoDB ever, or write any code for anything that touches it, be aware that you may be forced to open source anything and everything.