This is a pretty simple fix I'll list it out step by step so it's easy to follow
1. Right click the sG logo on the far left side of the screen
2. Click on "Edit Server Profile"
3. Type whatever name you'd like into the "Nickname" box
4. Press "Save Changes" on the pop-up
If you are still struggling with it afterwards, I can help you out with a screenshare, or step by step guide with images. Just shoot me a dm on discord Rock#0004