How to convert an executed MIS order to an CNC order using kite web application?

Go to positions, the position you intend to convert you will see a > mark. click on it, and from here you can convert the position.

Will this option be available even on mobile version?

Yep absolutely. Check this user manual

https://kite.trade/docs/kite/#kite-mobile

Can I convert MIS to CNC and CNC to MIS at the end of the day?

You can but you will have to before 3.20 PM. Visit Kite User Manual for more.