Yes, you can easily upgrade your plan at any time.

If you’ve chosen monthly billing, a new billing cycle will immediately start. Your remaining minutes, however, will not roll over. Therefore, it is often better to upgrade at the end of your current billing cycle.

If you chose for yearly billing, no new billing cycle will be started and you'll get a discount based on the price of your previous plan and the months that haven't been started.
For example, if you had a yearly plan that started on 15/02/2017 and ends on 15/02/2018 and you decide to upgrade on 13/04/2015 you'll pay 11/12 times the price of your new yearly plan minus 10/12 times the price of your old yearly plan.