multiple OS

Just remove the Windows 7 drive (to prevent any errors), and boot from your Windows 10 install medium to install Windows 10 on another hard disk. Then depending on which disk you boot from, you're in the OS on that disk.

But it's worth a try to first try a somewhat more elaborate way. Clone your Windows 7 disk to an empty one. Then remove the original one (to prevent any errors), boot into the cloned one (running Windows 7) and run setup.exe from your Windows 10 install medium. Might work and then you have all your programs and settings already available. But no guarantee because of drivers. But when it good enough to install all Windows 10 drivers from the makers site, you're fine. And if not, delete everything from that cloned disk and do a clean install of Windows 10.

Microsoft for decades tends to not support this area automatically so you must take care to not blow up an OS that you want to keep.

It's dated but the steps at https://www.howtogeek.com/197647/how-to-dual-boot-windows-10-with-windows-7-or-8/ seem to work today but you must use the advanced install and BACK UP what you can't lose. So many skip the BACK UP that it's no longer funny when the client complains they lost it all.

First is theis Windows 10 been activated on the same PC before? If it has been activated on another PC you could have problems. Now if you purchased this as OEM and it hasn't been installed or if it's a retail version you shouldn't have a problem.