I just did this recently with a dedicated server in RAID 6 where a disk failed. They have a page on their wiki to walk you through it, but basically you boot up

I just did this recently with a dedicated server in RAID 6 where a disk failed. They have a page on their wiki to walk you through it, but basically you boot up into the rescue system (network boot, activated on user panel). Then you figure out the failed disk based on existing or missing serial number, input that into the support form, request replacement. This was done in about 20m, then I rebuilt the RAID, rebooted it was fine