When you are editing a form in form builder, you have the ability to quickly edit the order of fields by using the Reorder feature.
To use this feature, follow these steps:
Go Forms on the left-hand menu
Click on the form that you'd like to edit
Click Edit
You will now see an option under Save Changes titled Reorder
You are now in Reorder mode
Scroll down until you see the all of the fields currently on the form
Simply move the fields around by dragging and dropping below or above until they are in your desired order
You can also drag and drop fields into and out of Field Groups. Please note that in order to add branching once you've moved a field into a Field Group via Reorder, you will need to go back to Edit Fields and set up the branching from there
From here, you can Save Changes to save the Reorder
If you need to make additional changes to the fields themselves, you can click on the Edit Fields button and it will take you back to edit/add fields
Be sure to save your changes by clicking Save Changes after all edits and Reorders are complete
See below for a quick video on how to use the Reorder feature:

