I have instantiated the nmos cells from the Device1 bar , and now want to edit (trying to delete the metal-poly) , but not able to do it with selecting and the erase * command. How do i make that portion editable ? Thanks

There are several answers to this question: (1) You can use the device GUI to remove the top or bottom contact from the gate, as that is a standard option with the FET device generator; and (2) You can always hand-modify any parameterized device, although be advised that if you select the device GUI and change a parameter from the GUI, then the device will be redrawn from scratch and your edits will be lost. The parameterized device is a cell, and to edit the cell contents in magic you have to select the cell for editing. Select the cell instance with the "i" key, and then either use the "e" key (or "edit" command) to edit in place, or use the ">" key to push into the device layout and edit it there (the "<" key pops back to the parent layout).

I tried the 'e' edit key and after that even 'd' deleting key , but the tool is saying "You selected one or more subcells that aren't children of the edit cell. Only those in the edit cell were modified."