Close Steam completely
Windows: open Settings
Windows: go to Apps
Windows: select Installed apps or Apps & features
Windows: find Steam
Windows: click Uninstall
Windows: confirm the uninstall
Mac: quit Steam
Mac: open the Applications folder
Mac: drag Steam to the Trash
Mac: empty the Trash
Optional: delete leftover Steam files from the user Library folder
Optional: delete the Steam folder in Program Files on Windows if it remains
Optional: restart your computer