If your unit is MY20, keep MY20 protection enabled, that prevents creating reformat USB.
And yes, it’s as simple as that: set your current build to your current build in the configuration and then select 3.4.21265 as the target.
Since your unit is NO-NAV, you may need to follow this guide for installing a new version Tutorial: Sync 3.4 NON-NAV APIM: Failure to Update to Newer Version Sync 3.4