Viewing your My Meetings page, click on your Profile on the upper left corner and see the pencil icon next to your Meeting URL. Click that icon.
Alternately you can find this under your Account, click on the Public Profile tab on your Account page.
For Meeting plans the following window will pop up:
Click Save and you are done!
Webinar Plans will be taken to your Public Profile page to edit your URL.
Please note the following information
If the meeting URL you chose is not available you will see a warning. Please choose another URL. For the Privacy of all customers, we ask that you do not contact Support to inquire about the URL you want being taken. Support cannot give you information about other accounts and cannot take the URL from another customer to provision your account.