All existing email signatures are displayed as tiles on the Home page. To change one of these, you can:
Hover your cursor over the thumbnail preview section to access an Edit signature option:
Select the options button to access the Edit 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.