Here is the procedure to change the username

1. Access Settings: Click on the top-right menu ("hamburger" menu ☰) and select Settings

2. Navigate to Your Account: On the Settings page, locate the Your Account section.

3. Click on (Change) button next to the username.

4. Type new username in the Change Username popup window and follow the instructions

Note: Username change will also change your stck website's links. There are limits on how many times one can change the username.
If you have custom domain mapped please read this help section.