This can be done using the Custom Script Assistant.
When you drag one of the pre-built forms onto your page, all items in the form will be "grouped" so you can drag the form around as a whole. It's easier to change items in the form if it's not "grouped", so right-click on the form and select "Ungroup" from the menu that appears.
To change the name of a field, you need to change the script attached to the field. Select a field, then go to the "Tools" menu and select "Custom Script Assistant". Select the "Object Events" button. In the "contents" box you will see something like this:
...you can change "Address" to whatever name you want, but we recommend using only upper and lower case letters, numbers and underscores - no spaces or other punctuation.
After altering the script you will also have to change the text label that appears next to the form field. This label can consist of any characters you want.