You will need the Limited Write Contributions permission to perform this action.
Click on "Contributions > Envelopes" in the left-hand navigation menu. In the table that appears, click on "Unassign" in the drop-down menu at the end of the row containing the envelope you want to remove. A confirmation message will appear. Click on "Yes" to finish.
Helpful Tip
Contributions added using the envelope number will not be deleted but will stay associated with the person.