This process involves a few steps within the built-in Disk Utility program, particularly when your partition is encrypted.
Before we begin, note that you should see hierarchy of disks/volumes in Disk Utility. At the highest level, you should see a logical volume group representing your external drive, with at least one Time Machine partition nested underneath it. These two items could both have identical names, e.g., "Time Machine," but they represent different things.
Also, beware that following these steps will delete everything on your external drive.
In Disk Utility, "unmount" (don't "eject") the Time Machine partition.
Erase the Time Machine partition.
Erase the logical volume group, i.e., the higher-level item that contains the partition.