1. Highlight the desired signature and press CTRL + C.
2. Login to OWA (Outlook Web Access) at https://outlook.office365.com/owa
3. Click on the icon that looks like a gear near the upper right section of the page, and then select the "View All Outlook Settings" option at the bottom of the window.
4. Click on the "Compose and Reply" section.
5. Paste your new signature in the window by pressing CTRL + V.
6. To make your new signature the default signature, select the option "Automatically include my signature on new messages that I compose".
7. Click "Save", and then click the small "X" next to the Save and Discard buttons to exit the settings screen.
8. When you create a new email, you will see your signature is automatically applied.