Locate the field you want to move within the Personal Custom Fields tile in Site Settings.
Then click on the arrow to the right of the field.
Choose the group you want to move the field to from the dropdown list then click on OK.
You can continue to select additional fields to move and then save the changes in one go.
You will see a status appear in the column next to the field type indicating that field has been set to move and which group it is being moved to.
To undo the move, click on the undo arrow.
To accept all moves, click on Save at the top of the page.
To discard all moves, click on the Discard Changes link at the top of the page.