The steps taken to replace a network card may vary depending on the manufacturer of the network card, and whenever possible the specific instructions provided by a card manufacturer should be followed. In general, however, replacing a network card typically involves uninstalling software that supports the existing card, then physically removing the card, installing new software for the replacement card, and then installing the network card inside the computer tower or case. If you are trying to replace a card with the exact same card, then you may not need to uninstall and reinstall the software that accompanies that card.
One of the first things you should consider as you try to replace a network card is that the replacement card needs to work with your computer and operating system (OS). You should also consider if you want to use an external or internal network card. External cards can be much easier to install and use, though they may not be as reliable and will require an external connection. To replace a card inside your computer, you will need to find a card that will work with your computer and OS and that can utilize a port you have free within your computer.
Regardless of whether you are trying to replace a network card inside or outside of your computer, you will likely need to first uninstall the software for the old network card. If you are replacing the card with the exact same card, then you may not need to do this as the software should simply recognize the new card as easily as it did the old one. Assuming you are trying to replace a network card with a different card, however, then once you uninstall the software you should shut down your computer. You can then physically remove the network card from your computer, either by unplugging an external card or by removing an internal card from the motherboard and computer case.
Once you remove the old network card, you can then turn your computer back on and wait for it to finish booting. You should then install the software for your new network card, and it will likely prompt you at a certain point to shut down your computer and install the new card. If it does not prompt you to do this, then simply wait for the software installation to finish and then shut down your computer. You can then install the new network card either by connecting it to your motherboard or by simply plugging an external device into your computer.
If you are trying to replace a network card with an external card, then you may not even need to shut down your computer before plugging in the new card. Start your computer back up, or wait for it to recognize the new external device, and the software you installed should finish installation and help you set up your new network card. This will typically include connecting to a local network or the Internet and helping you deal with any troubleshooting of issues encountered while trying to replace the network card.