dell,tower?

Do you mean the ethernet card?

Turn off the computer and unplug it.
Open the left side pannel.
Find the card, remove the retaining screw.
Lift the card out.
Insert the new card in place.
Secure it in place using the screw.
Close the case.
Reconnect and power on.

Windows should automaticaly identify and correctly configure it.

If it's a classical dialup modem, you probably need to install the required drivers.

Electro
November 2011