You may need to delete an allocated envelope from a donor.  This may be due because you have terminated them, they have stopped giving weekly or it was allocated to them incorrectly.

Navigate to the donor's details and within the envelope management system, click on the blue cross next to the envelope you wish to delete.

If there isn't an option to delete then it is most likely due to that donor having donations already against them (and the envelope) in that particular year.  If this is the case you may need to delete the donations against that donor also.