Tim Machine HD Icon

Or, you could simply hide the icon instead of changing it. Go to Finder > Settings > General, and uncheck "External disks" from showing on the Mac desktop.

Change Finder settings on Mac - Apple Support

Jack

Okay, so change the picture if you don't like it. If you don't know how here is an old article that explains how:

https://9to5mac.com/2021/11/08/change-mac-icons/

Where it says "Applications" just navigate to the Desktop and select the Time Machine icon. Of course you will need to find another icon that pleases you. There are thousands to select from, you will need to do your own research to find one that you like.

Where on the screen do you see it? In the Dock, on the Desktop, in the upper right of the display etc.... You must be more descriptive otherwise we cannot help you out.