Below we may get step-by-step screenshots,

Step 1 - Click on "Start" button.

Step 2 - Click on "All Programs".

Step 3 - Click on "Windows Update".

Step 4 - On the left pane, click on "Check for updates".

Step 5 - Download and install all the updates available for the existing version of Windows 7.

Step 6 - Make sure the installed copy of Windows 7 is activated by opening "Start | Control Panel | System and Security | System".

Step 7 - Click on "Internet Explorer" icon on the task bar to open the IE browser.

Step 8 - Visit the link - "https://www.microsoft.com/en-us/accessibility/windows10upgrade" and click on "UPGRADE NOW >" button.

Step 9 - Click on "Save" to store the upgrade tool on download location.

Step 10 - Click on "Run" to execute the upgrade tool.

Step 11 - UAC prompt will appear, read carefully, confirm and click on "Yes" to allow and start.

Step 12 - Please wait a while, Windows 10 Upgrade Assistant is checking for updates.

Step 13 - Read carefully and if you accept all the Terms & Conditions and wishes to continue upgrade process, click on "Accept".

Step 14 - Please wait a while, Windows 10 Upgrade Assistant is checking device compatibility.

Step 15 - Please wait a while, Windows 10 Upgrade Assistant is downloading Windows 10.

Step 16 - Please wait a while, Windows 10 Upgrade Assistant is installing Windows 10.

Step 17 - Perform restart when prompted and login, Windows 10 will set things up before showing Desktop screen.