You can still upgrade your Windows 7, 8 or 8.1 to Windows10 for free as of 1/6/2017,
Before starting I suggest you,
Remove any files not needed and then run a disk cleaner to remove all deleted and temp files.
Make sure your current OS is up to date with all Microsoft Patches.
Create an image of your current operating system using the builted in
Windows7 or 8 backup software (control, Backup and Restore, Create a system image).
(As of the anniversary update of Windows10, you only have 10 days to roll back to the previous version.
With the image you create now, you can roll back at any time.)
Be sure you have created a Windows7 or 8 Repair Disk so you can re-install the image you are creating.
After the download completes, run the Media Creation Tool.
If you've downloaded the Media Creation Tool on the machine you plan to upgrade,
and you plan to upgrade one and only one PC,
you can choose the Upgrade this PC now option and be done with it.
Follow the prompts to complete the upgrade.
You will not be asked for a product key, and when the upgrade is complete,
you'll have a digital license to Windows 10,
which you can confirm by going to Settings > Update & security > Activation.