Yes, you can cancel long-term plans by sending us a Support request. However, the discount you received for your long-term plan will be cancelled and you will be charged only for the period that you used.
For example, if you purchase a 3-month plan and receive a 15% discount for it, and you choose to cancel after one month, then you will be charged the full monthly price (what you would have paid if you didn't buy a 3-month plan). The remainder will be returned to you. Also, if you have started another month and chose to cancel your plan after a few days, the system will charge you for the new month even if you didn't finish the whole month.
Remember, you can always pause your subscription for 7, 14 or 21 days (max of one pause per subscription month). You can do it from your subscription page inside your student account.
Alternatively, you can also downgrade your plan to a smaller one and receive a refund for the difference in cost. Again, you can make changes from your subscription page.