You can do an in-place upgrade.
1) Insert W10 media with system running Windows 10 already (link to make is at the END if you need it)
2) run the "setup.exe" program on it
3) when prompted have it KEEP your programs and data (unless you decide to do a clean install in which case I'd BOOT to the media and DELETE everything off the Windows partition)
4) finish. I can't remember if it prompts for a key but if it does just SKIP that.
Basically this does a re-install of the core files.
Other:
Once you're happy it's running well you should:
a) right-click C-drive and do a cleanup of the system files (properties-> disk cleanup-> cleanup system files) and delete the "OLD" Windows 8 folder etc to save a lot of space (there's a backup in case you want to rollback to W8.1)
then
b) make a backup Image if you have a spare drive using Acronis True Image or similar
Other:
If you decide on a clean W10 install, you'd navigate to the MS media creation tool, download, run it then burn a DVD or USB as applicable. Then go into the BIOS and boot to it, delete partition data and continue (no need to enter key)
https://www.microsoft.com/en-ca/software-download/windows10