According to Parallels, "Customers who purchased and registered a copy of Parallels^® Desktop 17 for Mac from the Parallels online store (parallels.com) or an authorized reseller between August 1, 2022, and September 30, 2022, are eligible for an upgrade to Parallels Desktop 18 for Mac at no charge, subject to certain conditions."  For more information, please click here. 

If you have bought a perpetual (one-time purchase) Parallels Desktop for Mac license of any version prior to August 1, 2022, you can buy an upgrade license directly at the Parallels website here.