how can I change my username?

You need to request it to us. Please let us know what your desired username would be, and also provide 2 more alternate names in cased the proposed one is not available.

Just to let you know, if you had created custom URLs, you will need to create new ones, as they will not longer work once we change your username: https://www.jotform.com/help/261-How-to-Create-a-Custom-URL-of-a-Form