Redis Team has very good documentation on this
Core Steps:
- Setup your new Redis instance as a slave for your current Redis instance. In order to do so you need a different server, or a server that has enough RAM to keep two instances of Redis running at the same time.
- If you use a single server, make sure that the slave is started in a different port than the master instance, otherwise the slave will not be able to start at all.
- Wait for the replication initial synchronization to complete (check the slave log file).
- Make sure using INFO that there are the same number of keys in the master and in the slave. Check with redis-cli that the slave is working as you wish and is replying to your commands.
- Configure all your clients in order to use the new instance (that is, the slave).
- Once you are sure that the master is no longer receiving any query (you can check this with the MONITOR command), elect the slave to master using the SLAVEOF NO ONE command, and shut down your master.
Full Documentation:
Upgrading or restarting a Redis instance without downtime