Here is Garmin’s documentation about trimming an activity.
You will need to delete activity from CityStrides & Garmin Connect in order to get it reprocessed.
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Download the raw, original activity file from Garmin Connect and save to your computer. This is a backup in case anything else in the process goes wrong. This will likely be a .zip or .fit file type.
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Trim the activity on Garmin Connect and confirm it looks as you want it on Garmin Connect
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Download the new trimmed activity from Garmin Connect and save to your computer. This will be used later. Make sure you can tell the difference between this file & the file from step 1. Note: You may need to download the GPX version of activity (vs original). The GPX file may not include some of the advanced data analytics your device recorded during the activity.
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Delete the activity on CityStrides. This may take a little while to complete. Grab a beverage from fridge as you wait. It is critically important that this activity is removed from CityStrides before proceeding.
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Delete the trimmed activity from Garmin Connect
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Upload the trimmed activity file to Garmin Connect. This will cause Garmin Connect to re-process the run as a new activity with a new activity ID.
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Wait for CityStrides to pick up this new activity from Garmin Connect
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Confirm in CityStrides the activity appears trimmed as it did in Garmin Connect
Disclaimer: I have not had to do this myself so recommend trying with a throwaway activity first before doing for an activity you care more about.
NOTE: Do not deauthorize/disconnect access between Garmin & CityStrides as this will delete all your data