You can edit the farm or entity for a particular field in 2 steps:

Click on the 3 dots next to the name of the field you need to change and click Edit.

Then use the drop-down arrow to select the desired farm name. If you need to add a new farm, click on the "Add or edit farms" button below the dropdown. Be sure to click "Save" at the bottom when you are done.

Note that you will only be able to move a farm to an entity that is associated with your account.