You can get the free up-gradation from Windows 7 to Windows 10 by following the below steps.

Stage -1: Get media creation tool

Get at least 8GB USB drive and insert it into a USB slot. Then run the media creation tool to create the installer USB drive. 

You can get the latest MediaCreationTool1909.exe (versions may vary over time, but this is the latest one now) from https://www.microsoft.com/en-us/software-download/windows10. This will allow you to create windows 10 media ISO file which can be burned to the USB drive (CD can also be used).

Remember, booting from USB can be slow on older systems or if you are not using a USB 3.0 flash drive. 

Stage-2: Cleanup your Windows PC. 

• Open Windows Explorer. 
• Navigate to C:\Users. 
• Double click the account name you logged in as. e.g. :\users\John\
• Click in the address bar and add \appdata. It looks like this C:\users\John\appdata. Press Enter. (In case you cannot see app data make sure you unhide the hidden file by right-clicking in the folder and clicking on properties and uncheck hidden.) 
• Then click the Local folder, then open Temp folder. Now your address bar shows C:\Users\John\AppData\Local\Temp 
•  Then select everything in that folder and press the Delete key and click Yes. (you can select all by pressing Ctrl and A together or from the menu option). 
• You can skip any files which can’t be deleted. Do not worry.
• Then you can go one step back by clicking Local in the address bar. Then open Microsoft, and then open Windows and then open WER. Here, in ReportArchive and ReportQueue folders you should delete everything if they are not empty. You can do this by opening ReportArchive folder and then select all files inside and press delete key and then hit yes. You can then click the left (back) arrow or WER from the address bar then open ReportQueue. Here you can select all the files and press the delete and then yes.
• On the address bar, click Local Disk (C:) Click click in the address bar and add program data. it will look like this C:\ProgramData
• Again, open Microsoft, then open Windows, then open WER. Delete everything in ReportArchive and ReportQueue. If they are empty, move on. You can open ReportArchive and select all and delete them all. Then click the left (back) arrow or WER from the address bar then open ReportQueue. Select all and press the delete and then yes.
• On the address bar, click Local Disk (C:)
• Here you can open Windows then open the Temp folder
• Again select the contents and press the Delete button and click Yes. Some files may not be deleted. That is ok.
• On the Address bar, click computer
• Locate Local Disk C:. Right-click on this and select Properties (or highlight it and select File / Properties) Same thing.
• Click Disk Cleanup. Select all the boxes and then click Delete then Delete files. 
• Then go to your computer Recycle bin, which can be located Desktop, right-click on it and press Empty Recycle Bin ( or you can open recycle bin select all and press delete button and press yes. Both are the same thing). 

Stage-3: Upgrade to Windows 10

Now all the cleaning is done. Go go to the USB drive. Double click Setup.exe.

On the first screen, it will ask to Get Updates and Optional Features. Choose Not Right Now. Then press Next.

Now choose the appropriate options to finish the install. That is all.

Once Windows 10 installs, select No to all the sharing options offered during the configuration steps. Make sure to apply the latest updates once windows 10 is installed, which you can do it by going to the Windows Setting and choosing Windows updates.

 Note: the windows installer consumes a lot of time at least 70%. So, be patient.