I don't know your setup, but normally RAID 5 & 6 are hot swappable. Meaning you can pulpull the drive out while it is running. As you you don't know about that, your system probably doesn’t have that setup. The difference between RAID five and RAID 6 is how many parity drives you have. Usually RAID 5 has 1 drive and RAID 6 has 2. This means you can have 2 drives fail without data lose.
So I would follow these steps.
- verify that your RAID is RAID 6
- backup all your data to a external hard drives.
- power down your system.
- remove 1 drive
- reboot your system. You should get an error. but you should still have access to your files. if not put the drive back in. Your data should be fine as you did a backup.
- power down system add new drive. then boot the system.
- In the software add the drive to your volume. It may happen automatically, depends on your setup. The other drives will then replace the data missing from your old drive and copy it to the new one. This will take awhile.
- repeat the steps for the other drive.
Make sure your replacement drive is the same type s the old one