To access the form editor, click on the "Mobile Forms" button for the dataset with which you are working.
Next you will be looking at this interface:
At the top you can switch between the three types of mobile forms: table, edit and create. In this tutorial we will focus on the latter two.
On the left main panel you will see a bunch of HTML code. This is what gets rendered on the mobile device as a form. You can see the result on the right panel.
You can make any changes you want to the HTML on the left. By clicking on the "Refresh" button you will see the result of your changes on the right panel. This button, however, doesn't actually send your changes back to the server. Only by clicking on the "Save" button you will have made a permanent change to your dataset.