Hi there and welcome
to do this, first download and install Garmin Express if you do not have it already installed.
You can get it from my link below
Open it and sign in with your account and then click on Tools.
Then Remove Device and remove it
Then uninstall the connect app on your smartphone
Then install it again
Sign in on the connect app and you should not see the device
Then all data is cleared
If you have any problems, or would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.