extending travel credit

I am sure there is a thread for this but I can't find it.
I have a travel credit that is due to expire on May 12. I won't be able to use it by then...how can I extend it? Can I make a future reservation and cancel it to buy some time? Will I then get a new expiration date? And does travel need to take place before 5/12 or just be booked by then?
thanks & mods...move if necessary