Hi I have a Nec DT300 at work. I am on a new desk. Do you know how I can change the name which is on the display.

If you have the Admin password then follow this procedure:
For a DT330 (24-Button with 4 line LCD)
Do not lift the handset and press the Speaker button
Enter #*#*
Enter the 4 digit admin password and press the Transfer button
The phone will now display 'Program Mode'
Select Base from the four options using the associated softkey
Enter 15 and press the select softkey 2 times
The Extension Number and Ext Name will be displayed
Find the Extension Number you want to change the Extension Name for using the + and - softkeys
Press the Mic button to go to the Ext Name line
Enter the New Name using the number dial keypad.
If you make an error the Recall button will delete a character to the left and the Hold button will delete all characters to the right of the cursor.
Press the Transfer button to accept the new name.
Press the Answer button 4 times or until 'Program Mode' is displayed.
Press the Speaker button.
The system will save the changes and return the display to normal.

You'll have to talk to someone with administrative rights to the telephone.