This answer assumes a mdadm based Raid 1. It is a very general answer, as you haven't really provided much details on the actual setup.
To un-raid a software Raid 1 you do the following:
- Remove one of the disk/partitions from the raid device, putting it in a degraded mode.
- Create a fresh partition, filesystem etc on the freed drive. Rsync all files from the degraded raid device to the new partition.
- If /boot/ is handled by a raid device, repeat 1 and 2 on it as well.
- Fiddle with /boot/grub/menu.1st and /etc/fstab to make sure the system is booted without the degraded raid device(s).
- Clear away the degraded raid from the remaining disk.
(Preferable by first practicing on a local, possibly virtual, test system.)