Is it possible to change the number of reserved hours?

Yes.

First of all, note that the standard fair use policy allows you to exceed your reserved hours a little for those busy months. 

If you're consistently exceeding the reserved hours, it's worth upgrading to a higher plan as you'll get a better price per hour compared to overtime. Simply send us an email asking us to increase the hours; that will go into effect the next month. We'll invoice you the extra time separately, or as part of your next quarterly bill. 

If you've overbooked and want to cut down on the number of reserved hours instead, you can request that in the same way via email. However, as you will have already prepaid for a certain number of hours for the 3 months period, a downgrade will only go into the effect at the start of the next 3-month billing period. 

In both cases we ask for at least a 2 week notice before the month/quarter is over.