Very simply. If the old artwork was downloaded by iTunes, select the tracks, control click (or right click for Windows) to bring up menu, select Clear Downloaded Artwork menu item.
If the old artwork was user added, select the tracks, control click (or right click for Windows) to bring up menu, select Get Info menu item.
In the dialog, if you had selected multiple tracks, you will see an Artwork box on the Info page. Simply drag the new graphic file onto that box to replace the artwork.
If you had selected only a single track, you will not see an Artwork box but you will see an Artwork tag instead. Click that tag and delete the old artwork there.
If the old artwork was user added, select the tracks, control click (or right click for Windows) to bring up menu, select Get Info menu item.
In the dialog, if you had selected multiple tracks, you will see an Artwork box on the Info page. Simply drag the new graphic file onto that box to replace the artwork.
If you had selected only a single track, you will not see an Artwork box but you will see an Artwork tag instead. Click that tag and delete the old artwork there.