Home Designer can be used to easily fill in the exterior walls on a new floor. In fact, we will do that because it saves time and the need to align all the second floor walls to the first floor walls. We will still have to delete some walls and create some others, but overall this is by far the best approach. So, zoom out until you can see the entire plan, and select the Floor Tools icon (shown below).
Now, select the Create New Floor child tool (on the child toolbar to the right) and you see the following dialog box.

The proper option is selected by default, so click OK. The screen now changes to something like what's shown below.
But, where is the first floor? How can we see it in relation to this new floor? Glad you asked. Find the Reference Display tool, and click it to turn it on. You can also click F9 to accomplish this. Now, the display looks like this:
The first floor walls show in red. Switching between floors is easy, simply click the "Down One Floor" tool (the big < arrow), or the "Up One Floor" tool (the big > arrow).
Now we will start filling in the walls of the second floor.