No need to uninstall the old version.
You just have to launch the new version setup.
It will just ask to close the old version when it will be required.
There is no downtime (Collector and Fortigate) and no lost of logons during the upgrade as the new software will re-read logoncache.dat and groupcache.dat when it will start and as the operation is very quick the fortigate don' t have the time to flush logons so fortigate keeps all the already known logons information.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.