How to change WAN to 1000M from 100M?

> I have directly connected the LAN cable to desktop [...]

   What are the devices at each end of this cable?  Are they both
gigabit-capable?

> [...] I am getting 175 Mbps speed in the Speedtest. [...]

   Which "the Speedtest"?

> Inside the router administration page I am seeing WAN is 100M/full. I
> don't see any options to change this to 1000M or something. [...]

   There are no such options; the router negotiates its port speed with
the device at the other end of the cable.

> [...] I am sure that there is no issues in the cable. [...]

   Prove it.  As a test, if you connect one of your cables between two
LAN Ethernet ports on the R6400v2, then what do the corresponding
port-status LED indicators indicate?

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Read.  Look for the LED
descriptions.

   According to your picture, the LAN4 port also runs at 100Mb/s.  What
happens if you test that cable?

> [...] The cable is a four-pair cable only.

   All eight wires are needed for a gigabit connection.  Only (the
right) four are needed for lower speeds.