If you get a diagnostic "memory full - Need to sync to Garmin Connect" and you have already synced to Garmin Connect, this is how to fix it
Make sure that you synced all your activities with Garmin Connect
Connect your watch to your laptop USB port (I use a PC) using the charging cable
The Garmin device will show up under the "This PC" folder, as if a disk drive
Click on the Garmin device to open the folder
Under the Activities folder, either move the files present out of there (to save a copy to your PC) or delete all the files in this folder
Garmin - Please document this !