how do I erase timemachine and start new full backup

Follow the directions at > Erase and reformat a storage device in Disk Utility on Mac - Apple Support

Take your time and make sure that you are selecting the correct drive to be erased.

After the drive has been erased and formatted, Time Machine should recognize it and ask if you want to use the drive as your Time Machine backup drive. If it does not do that, please follow the steps in Back up your Mac with Time Machine to let Time Machine begin backing up.