You can update your signature at any point, either through the workrest app or web browser version.
To update your signature:
Select Menu ☰ (bottom right)
Then Account
Select the 'Update signature' button
Use your finger or mouse to create the signature in the space provided
Tap 'Save' at the bottom of the page
Old timesheets will still use the old signature, but timesheets from the point you've updated your signature on wards will use the new version.