Q: Where can I enable the settings?
A: After installing the addon, GA-Lite will be enabled by default. You turn off GA-Lite and re-enable the native Xenforo function simply by unchecking "Enable GA-Lite" in Admin -> Options -> Statistics & metrics
Q: If I choose to serve the file local, do I need to upload it somewhere?
A: No, if you change to the local hosted option, it will automatically point to the ga-lite.min.js file that is included in the addon package.
Q: What if there's something wrong? How will I know?
A: It is of course possible that your analytics data may be slightly different after changing to GA-Lite. Based on anectodal reports, GA-Lite works fine for the vast majority of users. There may be issues as noted in the description, in circumstances where you have configured advanced metrics or variables in Google Analytics, or are taking use of some features in the latest API. Again, you will know if this applies to you.
It's recommended you try using GA-Lite for several weeks, and monitor your metrics in your Google Analytics dashboard for any visible difference.