Once you click Edit on the Contribution Information block, there is an "Assign" link by Envelope Number.
Click "Assign" and a text box will appear, prefilled with a number which I believe is one higher than the highest in use. But, you can type in another number and click "Save".
If you enter a number already in use, it will not let you save and instead give the message: "The envelope number is assigned to another family."
I added a custom page under Contributions called Offering Envelopes List with two queries on it. One that lists all the envelope numbers in use with who they are assigned to, and another that lists a bunch of the unassigned envelope numbers, so I know what's available. Let me know if you'd like those queries - they are both short enough they can be run directly from the module settings and don't need stored procedures.