I forget to turn off Venu 3 and drive off after walking adding miles how do I adjust miles on watch?

You could try to Download the trimmed activity fit file from Garmin Connect and replace the fit file on the watch with the downloaded one.

The "export file" option downloads the original fit file. Trimming doesn't impact the fit file, only how the activity appears in Garmin Connect. Just now tested this, in case something had changed.

However, downloading as a TCX file does save the changes. Then the file would need to be converted to a fit file using a 3rd-party tool, then copied to the \GARMIN\NewFiles folder on the watch.

So it happen again driving home with watch on and GPS still tracking. With Fitbit once I altered the program the miles adjust correctly. What happens when I drive off after workout is that the Steps do not accumulated but miles and elevation does? Garmin should have an easy way to correct this on the watch. Connect shows the correct miles after trim. Maybe they can re-sync after trim back to the Venu 3 (watch)?

The Garmin sync process is one way. From the watch to Garmin Connect, regardless of any changes made in Garmin Connect. Personally, I don't find any need to maintain all of my activities on my watch, as long as it's on Connect.

I would like for Garmin to consider a feature request so when you use the Trim Activity and it corrects on the Connect App it will also reflect miles back to the Garmin watch (does not now). Fitbit has this capability but Garmin does not?

Stick a note on your car dash -Turn off Activity!