I am new to using RAID. However, at my new job I have to replace one of the drives in an IBM server because it's full. I have six HDDs total in it. 2 arrays, one of four discs, the other of two. How can I replace one of the discs for more storage without losing my RAID configuration? And what's the best way to determine my current RAID level?
"Replace one of the disks for more storage" doesn't sound like replacing a failed drive. If you want to reconfigure the RAID volume to ADD additional storage space then the procedures depend entirely on the RAID controller. You're going to need to find the documentation for it, or at least find out exactly what kind of RAID controller it is in the hopes that someone here is familiar with it.
Like sminlal said, adding more storage space to an existing array will come under 'Online Capacity Expansion' (OCE) if you look through the RAID controller's manual. 'Replace' is when a drive has failed which in this case it has not.
If this is a production server and I'm your superior, I'd be looking at someone else to do it while you look and learn.