With command:
cat /proc/mdstat
chech which disk is failed
poweroff machine, replace disk
copy partittion data from old to new disk
sfdisk -d /dev/sdb | sfdisk /dev/sda
add disk partition do raid
mdadm --manage /dev/md0 --add /dev/sda1
mdadm --manage /dev/md1 --add /dev/sda5
check resync progres
cat /proc/mdstat