If you need to add or change your Time Machine™️ backup drive, the process is pretty simple. Plug in a new disk and make sure there is nothing on it that you need. In most cases Time Machine will ERASE the disk before using it! You have been warned.
Now might be a good time to find the disk on your desktop and rename it to something obvious like “backups” for example.
Next, open Time Machine Preferences from the Time Machine menu. If you don’t have the Time Machine icon near the clock, you can also find the settings within “System Preferences”.
Click the “Use Disk” button, and Time Machine will start making a new full backup to this new disk. This may take a few hours.