It is quite simple to remove your Nissan Primastar radio. You will need a set of radio release keys (PC5-83). These are inexpensive and are around £3, you can obtain them from Amazon.
Once you have the radio release keys, push them into the 4 holes in each corner of the radio (See image), apply a little pressure and the radio will release from the dashboard. The radios label will be located on the radio's casing.
The Nissan Primastar radio serial is a letter followed by 3 digits for example N123. This is sometimes called the Security Code. This may be part of a larger serial/barcode number starting with 8200/7700/2811. An example of this would be 8200607918TJ109. Please see images below for reference.