back up

With the drive in question directly attached to the computer

Open Disk Utilities

1 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

2 - Choose the Upper Most Drive ( not the volumes indented and list below ).

3 - Look for the Name of the External Drive - example Samsung, Western Digital, etc.

4 - That is the drive to Erase and Format as APFS / GUID Partition Map ( for Big Sur usage ) or for Catalina and below- HFS Journaled and GUID Partition Map.

5 - Once formatted - ALL DATA on the Drive is Lost forever.

6 - Now, Start the Time Machine Backup Application and it should offer to use the newly formatted drive for that purpose - Time Machine Backup

Am sorry but the original quest was to Delete a Backup Drive to make a new one. There was a supplied method to perform that action.

One asked as to how to format the External Drive to be used on Windows. That too was offered a solution.

Now - the user wrote " Thanks a lot, but I have delete the hard with my Mac through disk utiility and now it is not readable on windows. " - where is this all going ??

Format for only windows as fat32 or exFat with the master boot records. For both apple and windows exFat