As Manish said it is not possible to extend that period, what you can do is once you install CUCM take a VM snapshot of the machine and at the time you hit 60 days mark revert to snapshot to keep it working.
(Rate if it helps)
If you goto cisco.com/go/licenses, you can sign up for a demo license for 6 months... I did recently for cucm 9x for my lab --- It was good for a period from 01/16/2017-07/15/2017.
I agree with Gregory, and now that I think about it, we have a couple of ways...
1) Buy an NFR kit.
2) 60 day default license (comes by default on 9.x). Load your vm and take a snap shot. After 60 days, restore the snap shot.
3) Load your vm with the 6 month trial, (cisco.com/go/license) and click on demo/eval licenses, and take a snapshot. After 6 months, you can restore or get a new trial license...