First question I have for you is why do you need/want Windows 7? In most cases downgrading to Windows 7 is a really bad idea, it's a dying operating system, your games aren't going to run slow on 10, 99.9% of programs work on Windows 10 that worked on 7/8.. unless you have a specific reason such as your a Windows 7 software developer (and again.. why??) I would keep Windows 10.
That being said...
Windows 10 can always be reinstalled via Flash Drive or DVD disc as long as you keep your serial number handy. If your laptop does not have the serial number attached to it (sticker on bottom), use a program such as
ShowKeyPlus to obtain it, write it down and keep it in safe place. Once you have your serial number you can always use the
Windows Media Creation Tool to create a bootable DVD/USB drive to reinstall. (I suggest making one before you go mucking about just to have it handy).
The other issue your going to have is your going to need to purchase a valid Windows 7 license (if you don't already have one), good luck with that.. some people have good success on eBay or similar sites.
You could also dual-boot Windows 7/10, I'd recommend starting fresh and creating two partitions, one for 7 and one for 10. Install 10, then install 7.
Hopefully this has answered some of your questions.