Q: Can I pay for Plesk license yearly?
A: Only monthly lease and 1-year lease licenses are available for purchase at https://www.plesk.com/pricing/
Q: I have a monthly lease license purchased from Plesk Online Store. Can I pay for it on yearly basis or pay for it once in 2, 3 or more months?
A: License renewal cycle cannot be switched from monthly lease to yearly lease, or back. 2, 3 or more months renewal cycle is not supported. Switch between 1-Year, 2-Year, and 3-Year cycles is neither supported.
If you want to change billing period for your subscription, you would need to cancel your current license (as described in the article) and purchase a new one with the required billing cycle at https://www.plesk.com/pricing
Purchase process is described here.