How do I go back to using the defult operating system on my pc, if I am booted up with a different OS saved on a flash drive?

I was super excited to try to boot up the Linux software Ubuntu on my Samsung PC that I had saved to my flash drive, I did all this research to do it right, and I got it to work, but I am borrowing this laptop.

My question is, how do retu the PC back to using the default Windows 10 OS, not the Ubuntu Linux OS I am currently using, without damaging either so they can be used in the future?

I would really appreciate help.