If you aren't already doing it, then better develop a habit to have a regular backup of your Mac OS X on an external disk. This way you can be sure of not losing your data, in case anything goes wrong. But if you are already taking a regular backup, and for any reason wish to change the backup disk in Time Machine or delete the existing disk, then it is very much possible.
It is also worth nothing that you don't need your existing backup disk attached to your Mac while you remove it from Time Machine. Also, removing a backup disk does not delete any data on the disk. The Mac simply stops backing up future data on the said disk. So now let's proceed with the guide, as you have got the idea of things.
How to Remove Backup Disk from Time Machine in Mac OS X
Step #1. Go to System Preferences by pulling down the Apple Menu on your Mac.
Step #2. Under System Preferences, go to Time Machine preference panel.
Step #3. Scroll down the list of options and click on “Add or Remove Backup Disk..”
Step #4. You'll now see the list of backup disks. Click on the one you wish to delete and then, click on “Remove Disk.”
Step #5. You'll be prompted to confirm the change, click on “Stop Using This Disk.”
That's it; now even if you connect the deleted drive to your Mac, it won't start backing up automatically, which it used to do earlier.