Striped (RAID 0) or JBOD, your best bet is to copy all the data off the device onto an external drive/device of sufficient capacity. Pull out all your old drives drives, install new drives, create array, copy data back.
Mirrored (RAID 1) or SHR - Pop out the bad drive. Put in new drive. Repair Array. Pop out second drive, put in new drive, Repair Array. Done.