Windows 10 Reinstall in an existing installation step by step Guide

Now you will want to launch the Windows 10 install from media.
If you are installing on a new hard drive then the install will automatically begin, you may though need to access the BIOS to set up your boot priority to include the Windows DVD-Rom or USB drive before the hard drive or just to enable their selection. See manufacturers instructions on how to do this. Select the device where you will be booting from

The installation will begin without interaction.

Now language selection, make selection and click "Next".

Now click "Install Now"

Depending on the Windows 10. It will have a different screen. Insert your product key. If your PC came with Windows 10 select Skip or Do this later. It will activate the license by itself if the PC came with Windows 10. Windows will detect that 10 is already registered and activate Windows for you. Check with your manufacture if your Windows is not activated after installation.

Accept the License Agreement

Types of installation. Upgrade will find a previous version currently installed and will attempt to upgrade to Windows 10. We will not select upgrade. Select custom to select the partition where you want to install Windows.
Now select the partition where you need to install Windows.
-Partition 1: System reserved
-Partition 2 will show as primary (Current OS is Installed here)
if you want to remove the contents of the hard drive, select the correct partition and click on Format or delete, this will erase the contents.

Options:
Format: You will now be warned that all data will be lost, click OK to continue.
No Format: You are now told that previous Windows on the drive will be copied to Windows.old document(This will save all current documents in the previous Operating system). This is great for people that have not backup their data. Click OK to continue.

Windows will begin the install process....then it will restart

Windows will reboot and finish with installation.

If you haven't entered a key it will ask you again, click "Skip" or enter key and click "Next" to continue...

Now the personalization settings, if you are happy with Microsoft's suggestions click "Use Express Settings" otherwise click "Customize settings"

Some of the Customize settings are speech, handwriting, location, delivered advertising settings, browser, protection, error reporting and connectivity settings. Turn on or off as you wish. click next after.

Now who owns the computer. Either is own by someone or a company(add computer to a domain), select and then "Next" to continue

Now one important thing to do, "the login" depending on the version of Windows we are installing the screen might be different. login with a Microsoft Account, if you don't have a Microsoft account click on "Create Account" or you can skip by clicking on "Skip" and create a local account.

Create local username and password. Recommend to write down password. If password is forgotten you will not be able to enter. Local accounts dont allow to recover the password.

Final steps Pin and Cortana are optional. After setting optional options Windows will finish the installation.

Done, dont forget to check and update your drivers.