I had Windows 8 and I format to win 7 I want to return to Win 8 + multiboot

[negevasaf]

I had Windows 8 and I format to win 7 I want to return to Win 8,
I still have the serial code of Windows 8 how can I return to windows 8?
I want to do multiboot with Win 7 & 8.1,
I already made The partition.

 

[negevasaf]

I forgot to say all versions are 64bit