Is there an Option to Edit the already created SLA Rules?

Service Desk does not provide an option to edit all the values in the SLA. This is because there could be active service desk records which would match the SLA that is being edited. The only option is to define a new SLA (if needed the original SLA can be de-activated using the SLA listing screen).

Please refer ‘SLA Activate’ section in help link SLA for configuration.