From its size in the screenshot I think that drive is standalone, not a member or RAID group?
In case the drive is not a member or RAID, it could not be “replaced” because the data on it will be lost after changing the drives.
If you just wants to change the hard drive after manual backup files, use “Detach” to remove it from the NAS safely, pull it out from the tray and plug a new hard drive to it.