hi everyone if I want to upgrade my fleet version from 3.5.1, what is the version that you recommend to upgrade?

You can go from 3.5.1 to the latest without any issue.

there are any big changes from 3.5.1 to the latest version that can hurt my fleet?

You can track the changes in the changelog here: https://github.com/fleetdm/fleet/blob/main/CHANGELOG.md