You have to purchase licenses for new version of vsphere. Example if you are having license for ESXi 5.x ,it can be used for version 5.0/.1/5.5 not for 6.x, you have to upgrade your license. Contact vmware license for pricing and offers.
Regarding upgrade path, if you have only vCenter and ESXi servers then the sequence would be , vCenter server and its components (PSC, vCenter server, Update manager) once you complete them you can start upgrading the ESXi servers using update manager or other upgrade options.
If you have any other solution integrated with vCenter like (SRM/ vDP or other solutions) , please check the respective solution guide /KB to find the sequence.
following link will be helpful for you to find more details
VMware Product Interoperability Matrices select upgrade path in the page and then solution