After a Diploma, you need to again give JEE Main and Advanced and follow the similar route like your peers in 10+2 stream. There is no provision for any such Lateral Entry in IITs. Post aceing JEE Main you get to appear in JEE Advanced, qualifying which gives you an opportunity to secure a seat at one of the IITs through the General JOSAA Counselling, where you may even change your Stream of Study if required.

For more information do follow the link below

https://engineering.careers360.com/articles/how-get-admission-in-iits-after-diploma

For more information about JEE Advanced do follow the link below

https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria