The link only states the products but I need to know the process to obtain Azadirachtin from neem oil or neem kernel /cake left after extraction of oil
If you are looking for procedures to extract azadirachtin from neem kernels, you may check the publications of Dr. A. Sankaram of CSIR-Indian Institute of Chemical Technology (formerly Regional Research Laboratory), Hyderabad, India. He has worked extensively on this subject for several years.
As far as increasing the content once the oil has been extracted is concerned, I am afraid that it is not possible since the azadirachtin content of the isolated oil is fixed. However, you can prepare products with different percentages of azadirachtin by concentrating the products. Such natural insecticides are available in the market.
If you require high azadirachtin yielding neem trees, one method is to screen several trees for oil as well as azadirachtin contents and identify plus trees. Even in such cases, there is no guarantee that the azadirachtin content remains constant over time as seasonal variations in phytochemicals is well-known.