If you connect your watch via the cable to the PC, you should see it as a drive in the file explorer.
I generally make a copy of it before changing anything. You can just make a copy as you do with any other folder and save it somewhere on the computer hard drive. This is also very useful if you have to reset the device at some stage as you can get activity screen settings copied back. (see link below)
Then you go to the correct subfolder (Activity) on the device drive (in file explorer) and delete the activities there.
This should delete the activities from the watch to give more space. It wont delete from Garmin Connect or Website if you do this, only on the watch.
Then unplug watch and do a soft reset (hold in light button until the watch shuts down completely, then restart after say 15 seconds)
I hope that helps. I am not sure what you are trying to achieve , other than making space on the device?
