how do i replace a failing drive?, raid 5 on omv6x

i have openmediavault 6.9.16-1 (Shaitan) running using a Buffalo nas box. for the system i am using an USB drive. then i have 4x 4tb drives in raid 5 configuration about half full with media. today i got a warning about a S.M.A.R.T failure on one drive, when i go to check sure enough i see under devices that SDA has a few bad sectors, i guess its time to replace.

i need to know how to go about this, first i will get an external drive and backup my data. but how do i replace to rebuild the raid or do i have to wipe and start over?

drives:

/dev/sdb ST4000NC000-1FR168 ATA Z302DACX 3.64 TiB 37 °C Good

/dev/sda ST4000NC000-1FR168 ATA Z303E25A 3.64 TiB 32 °C Warning

/dev/sdd ST4000NC000-1FR168 ATA Z303E3A7 3.64 TiB 35 °C Good

/dev/sdc ST4000NC000-1FR168 ATA Z303ECEL 3.64 TiB 37 °C Good

/dev/sde Tech JMicron DD56419883893 238.47 GiB Unknown n this is my system drive, an ssd drive.

in the end, OMV removed the failed drive from the array. i was then able to replace the drive, use a deep wipe to prepare it and then a rebuild to bring the array back to normal. no data loss that i can detect.

however, i got this message the next day:

A SparesMissing event had been detected on md device /dev/md0.

Faithfully yours, etc.

P.S. The /proc/mdstat file currently contains the following:

Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10]

md0 : active raid5 sda[4] sdb[1] sdd[3] sdc[2]

11720658432 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/4] [UUUU]

bitmap: 0/30 pages [0KB], 65536KB chunk

unused devices: <none>

is this something i can fix via the GUI, if not, could someone post steps?