All existing email signatures are displayed as tiles on the Home page. To change the design of one of these, hover your cursor over the required signature tile and select the Signature design option:
Having chosen to edit an email signature, it is opened in the Signature Editor:
All email signature elements and associated options are available in the left-hand pane. As you change these options, the signature preview in the right-hand pane is updated in real time.
Having made the required adjustments, click OK to save your changes and exit back to the Home page.