The guide at Upgrade Grafana | Grafana Labs is pretty comprehensive, and also details breaking changes that you may want to take into account. Whether you have a smooth upgrade or not will depend on things like whether all your plugins are compatible with v7 or not, which is obviously hard to judge from a distance. The best thing to do is probably to run a newer Grafana version on a clone of your production Grafana database, and see if it’s working or not.
As to which version to upgrade to, there’s usually no reason not to go with the latest - 7.5.3 as of now. My observation is that, occasionally, small regressions sneak into new major or minor releases (7.4.0, 7.5.0), but they get cleaned up pretty quickly with the subsequent patch releases.