If you're trading at Zerodha, you can convert your position from the Admin position window. Here's a screenshot of the Admin position window from Pi. You've to choose the position you wish to convert and click on the button.
Yep, If You are Using PI, Go to "Positions" and then "Admin Position". In "Admin Position" Select the Position and Click on "Convert Positions" which is Placed on Bottom. Almost same for the Web Portal and Click on "Partial Position Convert".