Where? On the watch, or in Garmin Connect? If on the watch, then either through the watch menu » History » Options » Delete All Activities, or from a computer through the USB cable, by deleting the files in the folder //GARMIN/Activity. If you want to remove all activities from Garmin Connect, and have a lot of them, then the fastest way would be creating a new account. You can also delete them individually (it is rather quick and easy in GC web), or in bulk by using a javascript code in browser's DevTools console.
