Locate the Online Backup app in the Finder. Most apps are in your Applications folder, which you can open by clicking Applications in the sidebar of any Finder window. Or use Spotlight to find the Online Backup app, then press and hold the Command (⌘) key while double-clicking the app in Spotlight.
Drag the app to the Trash, or select the app and choose File > Move to Trash
If you're asked for a user name and password, enter the name and password of an administrator account on your Mac. This is probably the name and password you use to log in to your Mac.
To delete the app permanently, choose Finder > Empty Trash.