how to resume SLA after re-open a ticket.
I believe this is not currently possible as by design, once a ticket is closed, the SLAs are "cancelled" - so you would have perform the same action or update to the ticket that triggers your SLAs to kick off again.
What is the situation that is resulting in a re-opened ticket? If the user is re-opening because the incident is not resolved, have you considered setting the ticket to resolved and then setting up auto-close after a period of time? This would give the user time to notify support if the issue needs more work (and the SLA clock would be able to pick up where it left off, since the ticket has not reached the closed/inactive state yet). Beyond this, it is generally considered best practice to open a new ticket.