Download it, if you haven't already, and follow the procedure in the D22 service manual. I had to get my injector pump replaced early this year. I didn't do it myself (that was before I joined this forum and found the service manual). With the help of the manual I reckon I would now attempt it myself. It's a pretty big job though- about 8-9 hrs to replace the pump for a mechanic with a fully equipped workshop. There is alot of stuff to remove before you can get the timing cover off and the pump out. Also, one of the main things is to lock the idler gear before removing the cam sprocket, timing chain & pump gear otherwise it's a nightmare to get the timing correct when putting the pump back in.
I think there might also be a special service tool needed. Not sure if you can buy it from Nissan, or make one up or just improvise...
You should also replace all the gaskets & seals (rocker cover, throttle body, glow plugs etc.) with new ones.
In the D22 service manual (D22.zip) open the following folders: D22Navara, then sm, then SM1E-1D22FG1 and open the file titled ec.pdf. The section on injector pump removal and replacement starts on page 762/1038.