How to change the label to make Windows 7 to be a default on this screen?

If you edit /etc/default/grub and edit two entries, it'll remember the last OS you selected to boot as the default choice for next time.
In terminal...
gksudo gedit /etc/default/grub
edit in these, near the top of the file...
GRUB_DEFAULT=saved # change this line
GRUB_SAVEDEFAULT=true # add this line
the first one is already there, but needs an edit. The second one you'll need to add. Then, still in terminal...
sudo update-grub