Do you mean your modem or your ethernet card?
The modem connect to the phone jack on the wall using a relatively thin, flat cable, normaly gray, and a small plug. It also have a bypass plug where you can connect your telephone.
The ethernet card have a similar connector, but larger, with more connectors and the cable is thicker and round. The cable can be any colour and should have "cat 5" or "cat 7" printed on it.
Both are available for prety cheap, usualy under $20.
Tool needed: A philips screw driver.
To replace any of them, do as follow (for a desktop model or tower):
Turn the computer OFF and unplug the power cable.
Open the case, wait for the LED inside to turn OFF. There are a few screws (3 or 4, usualy) to remove and slide the side pannel out. On a tower, it's the LEFT side pannel.
Locate the card to replace and unplug any cable connected into it.
Remove the screw securing it to the back of the case and lift the card out. Put it away.
Insert the new card and secure it in place with the srew you just removed.
Plug back the cable you previously disconnected.
Close the case, screw it back.
Reconnect the power cable and turn the computer back on.
Finish.
In the case of a modem, you probably need to install some drivers.
For an ethernet card, you normaly don't need to install any driver.
Electro, October 2011