Eddie51,
If you have a desktop PC then swapping hard disks is usually very easy.
Case designs do vary but you just take off the side cover(s) to see the hard disk.
It will be connected by two cables, a power cable and a data cable. Remove them (note they only reconnect one way so don't force them back in when you reconnect).
Unscrew the 2 or 4 screws that hold in the hard disk (depending on case design).
Take out the "old" hard disk and put in the "new" one.
Note some cases have room for two hard disk so you could put BOTH hard disks in, and then it is a simple job to move both cables from one hard disk to the other.
You can download Windows 10 from here:
https://www.microsoft.com/en-gb/software-download/windows10
You need to then put it on DVD or USB flash drive.
I have found it does not fit on a normal DVD (too large?) so maybe you need a dual layer DVD.
However putting it on a USB flash drive is the other option, as long as your PC can boot from a USB drive.
Make sure you have the EMPTY hard disk plugged in when installing Windows 10 or you could accidentally overwrite your Windows 7 disk !
Putting back in the Windows 7 is hard disk is just a reverse of taking it out.