back up disk in time machine

Select the old drive in the list and click the "-" icon to remove it. But you could also leave it in the list in case you want to reconnect it in the future to retrieve old backups. Personally, I'd just remove it to make things clearer in the settings of Time Machine.

Click on to highlight the backup2 and then click the -

+1 to 6x6 excellent advice

Eject the old drive and keep it in case you need to retrieve files from very old backups.

Connect the new drive to your Mac and set it in Time Machine. Time Machine will then do a full backup on the new drive and start doing incremental backups in the future (just like your old drive.)