Outplay automatically fetches Email Signature from your Gmail/Outlook on connecting mailbox.
If you still want to edit/update email signature
Click User settings.
Click Mailbox under User settings.
select Signature and paste your signature in the textbox.
Click Save to save changes.
Note : If you're facing trouble adding signature to outplay. Try adding signature to your GMail/Outlook mail and reconnect your outplay mailbox to the respective GMail/Outlook mailbox again.