NAS Raid 10 to raid 5

Unless your RAID card supports making this change "on the fly", you would need to back off the data and recreate the array. After this, you would see about 10.8Tb of space. RAID 5 would, however, be somewhat slower, particularly for writes, and more prone to data loss.

RAID5 uses distributed parity, so one the capacity of one drive is lost. Probaby better to refer you elsewhere than try to explain it here, there are lots of nice diagrams on the web covering this.

https://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5

So the current configuration is the optimal ?

Depends on what you want it to do. Migrating to RAID5 would give you more space, but less performance, and less reliability.

When a RAID5 array looses a drive, the replacement needs to be "rebuilt" from data on the remaining drives. This involves a heap of work, every sector on each drive needs to be read, parity calculated, and the new data written. It could take a couple of days with large drives like yours. Usually this process works, however it is not unknown for a second drive to fail during the raid rebuild. If that occurs, data is lost for good.  

RAID5 is thus less popular these days, larger drives mean a longer rebuild time, and drives nowadays are cheap enough to just go for RAID10 instead. I would only use RAID5 for a server that is somewhat expendable, rather than mission critical.

Redundancy & performance  are top priority .

If one drives fail on raid 10 what happens?

If one drive ina RAID10 fails, it can be replaced, and the data copied automatically from the other drive in the pair. This can still fail, however it is a lot more simple and less prone to errors than rebuilding a RAID5 array. Converting to RAID5 woud be a step backwards in terms of both reliability and performance, but would let you have more space.

Space isn't an issue.

So if one hard drive fails , I can just replace that drive ?

if space is not an issue, remain with RAID10 as others suggested and read this:

Its all about statistics and error rates in large hard drives why you should avoid RAID5.

http://www.zdnet.com/article/why-raid-5-stops-working-in-2009/