You can delete Ranks after they are created.

Go to Company Setup>Organizational Structure>Ranks.

Ranks can be deleted one at a time or simultaneously. To delete one at a time, tick the box of the Rank you would like to delete and click the Trash button next to the Add Rank button.

A warning prompt will appear.

Click Yes. Otherwise, click No.

You will be led back to the Ranks page and the deleted Rank will no longer be visible.  

If you want to delete all Ranks simultaneously, click the tick box at the first column and a Trash button will appear next to the Add Rank button.  All of the boxes will be ticked.  You can untick the boxes for the Ranks you do not want to delete.  Click the Trash button.

You are ready now to add a new Rank.