Regular users can only change their name once every 90 days. Patreon members can change their names once every 30 days.
To change your user name, start by clicking on your user name in the top right corner of the website, then click the account details link:
If you're on a mobile device, tap the hamburger menu (the three lines in the top left corner):
This will open the main menu for navigating the website. Next, tap on the person icon as shown below:
Here you'll see a list of links where you can see various information about your account. Tap on the account details link to continue.
If you're still having trouble, here's a direct link to the page:
At the top of this page, you will see your current user name with a change button next to it. Click on that button.
In the window that appears, type in your new user name, the reason you are making the change, and your current password:
When finished, click the save button and you are done. A moderator will need to approve your request before it becomes active.