"rename field" with the keyboard

Do you mean with only the keyboard? I don't believe that's possible, since you can't select the field in the first place. If you were able to do it (in Prepare Form mode), then you could use the Right-Click key, select R, and then type the new name. Another option is to click into the Fields List panel on the right, use the arrows to move up and down the list and when you're on the field you want to rename press F2 and enter the new name.