To change your username a second time on Steam, you can do so by following these steps:
Open the Steam desktop app and hover over your username at the top of the window. Select "Profile" from the drop-down menu.
On your profile page, navigate to the right sidebar and select "Edit Profile".
On the next page, find the "Profile Name" form and enter the new username you want to use. When you're done, scroll down and select Save (s**e) at the bottom of the page.
The steps are pretty much the same on the mobile app. Open the Steam mobile app and select the hamburger icon in the top left corner. Select "You & Friends" in the sidebar that pops up, then select "Profile". On the profile page, select "Edit Profile" and under "Profile Name" delete the existing name and replace it with the new username. When you're done, don't forget to scroll down to the bottom of the page and select "Save" (s**e).
It's worth noting that you can change your username displayed to friends and other players on Steam at any time, but your Steam account name is a fixed numeric identifier and cannot be changed. The Steam account name is assigned when the account is created and is used to log in to Steam, and even Steam staff cannot change the name.