A student was tardy 2nd and 4th hour, and is now tardy in my 7th hour - there is no place to mark another tardy - do we click on the "out" field and change the previous tardy?

No.  When there is already an attendance code in the Out field and you need to enter an additional one for a Tardy or a Skipped, you should click in the Out field and choose Post New Tardy-To-Class if the student is Tardy or choose Post New/Multiple Checkout(s) if the student is Skipping.  If you choose Post New Tardy-To-Class, you will be prompted to choose the Reason and E or U for the Tardy.  If you choose Post New/Multiple Checkout(s),  you will be need to choose 08 for Skipped from the list of attendance codes.