You can technically dissolve Raid10 by removing 1 drive, or 1 drive from each groups (not two from one group though which would terminate all).
I highly suggest to avoid this route, there is reason why NAS/SAS devices usually don't allow downgrading to lower Raid type directly.
Do you also have any external backup? You should :- ) But even if you don't, get one, and transfer files to it. Then use that to rebuild future storage accomodation.
I would even advise forgetting RAID exists, or at least, RAID 0 :- ).