Once you have fitness data uploaded to your Garmin Connect account, you will have options to edit the activity including the option to delete it.

To make changes to an activity in the Garmin Connect app choose your platform below:

Android

Select Menu (3 horizontal lines in top left)
Select Activities
Select All Activities
Select the activity you would like to edit
Select
Select Edit Activity (you can also choose to get rid of the activity by selecting Delete)
Make the desired edits to the activity details
Select Back Arrow in the top left of screen to save

iOS

Select More
Select Activities
Select All Activities
Select the activity you would like to edit
Select
Select Edit Activity (you can also choose to get rid of the activity by selecting Delete)
Make the desired edits to the activity details
Select Save in the top right of screen

Your activities will now reflect the saved edits in Garmin Connect. Please see related content for instructions on editing activities from a web browser on Garmin Connect.