I want to do a degree in aeronautical engineering, I am doing a Diploma in instrumentation engineering.

An aeronautical engineer work with aircraft. Their primary job is to design aircraft and propulsion systems but with time, the engineers are given many more responsibilities to carry out. The aeronautical engineers study the mechanisms of aircraft. They work for a career in aircraft, airplanes, JET

No you're not eligible for it. Diploma holders in aircraft maintenance engineering can acquire degrees on clearing the exams of the Aeronautical Society of India or if you did PUC based on JEE or JEE Main scores you can do.