Best way to upgrade from old Parallels Desktop (2010) and from Windows XP to Windows 7?

[ChristophB2]

Hello,
I have the following situation:
Very old Parallels Desktop (Year 2010) and Windows XP. Recently upgraded Mac OS X to El Capitan.
Need to upgrade both Parallels and Windows.

Q:
What is the best way to upgrade both Parallels Desktop to 12 and Windows XP to Windows 7?
First upgrade Parallels or Windows 7?

Thanks for advise!