If you need to reconfigure your mdadm RAID array, then it is probably easies to destroy it and then rebuild a new one. The following steps will show you how to destroy your existing mdadm RAID array.
WARNING: Make sure to move all of your data off the array before proceeding.
Stop the RAID array so that you can operate on it.
sudo mdadm --stop /dev/md0
Query your arrays to find out what disks are contained using
sudo mdadm --detail /dev/md0
Zero the superblock FOR EACH drive (or use gparted to delete the partitions if you have a GUI)
sudo mdadm --zero-superblock /dev/sdx sudo mdadm --zero-superblock /dev/sdy
You may also want to move your mdadm config to a backup file just in case.
sudo mv /etc/mdadm/mdadm.conf /etc/mdadm/mdadm.conf.bak