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 3-dots menu at the beginning of the row containing the envelope 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.