First you need to go here
Use the Download Tool to acquire the latest ISO and save it to a folder on your computer.
It is very important that you obtain the ISO that matches your installation (32 or 64 bit) AND in some rare cases we see from time to time a "single language" install will require "single language" media to successfully upgrade.
Then disconnect any attached peripheral, leaving only a USB connected Mouse, a USB connect Keyboard and of course your monitor. Nothing else.
Disconnect from the network, even if you have to go into device manager and disable your network adapter(s).
Uninstall any third party software security suites (antivirus and or firewall software)
Empty the entire contents of the C:\Windows\SoftwareDistribution
folder (everything, all subfolders and files).
In order to do this, you will need to run Services.msc
and stop both the Windows Update Services as well as the Cryptographic Service.
If you still have a Windows.old folder get rid of that as well.
You can do that relatively quickly by using an admin command prompt and typing
hit enter and check every box
Let it run till finished.
Go to the folder where you saved the ISO and double click the ISO which will mount it as a virtual drive.
Run Setup.exe from there to perform the inplace upgrade.