I have a level 4 v2.9.23 RouterOS and I want to do version upgrade to RouterOS 2.9.51. It is not working!

First confirm that you're license allows upgrading:

/system > license > print (check for "upgradable-to:" entry)

If it does allow upgrading, follow:
http://wiki.mikrotik.com/wiki/Upgrading_RouterOS

If not, purchase a license that does or take it up with your retailer.

Cheers.

Upgrade to 2.9.38 and next to 2.9.51.

Upgrade to 2.9.38 and next to 2.9.51.

not needed, if it works at all, it will work straight to 2.9.51 as well

Upgrade to 2.9.38 and next to 2.9.51.

I suggest this as well, not so much for the licensing issue, but for configuration saving purposes.