There are a number of ways to view archived strips, but here is one popular way: first go to www.gocomics.com and log in using your username and password.
On the next page, click on "Comics," toward the top. On the next page, click on "A to Z listing," then click on the comic you want to view the archive for.
Once the comic loads, look above the comic for your archive buttons. You can either click the small calendar icon to open up a calendar, or you can use the "left and right" arrows to go back or forward.