you should not forget to make backups of your data, configs etc.
You have to stop your splunk instance to make an upgrade, so there will be downtime. When the license master is down, you have a 72h period of time until the search is blocked on the slaves (indexing still continues). So there should be no problem with your invironment during the upgrade process. Then you should simply overwrite your master with the version you want. Everything is also documented here.
Edit: Here you can get information about the 72h window: Docu