You may have to apply for Lateral entry if you wish to pursue a degree in engineering. Now every state has its own admission process. If you could tell us which state you belong to, we can get back with better information. I do think you can change from Mechanical to Automibile but this also I can tell if you specify which colleges you are looking at. As for any courses, I cannot specify anything unless I know which area you may be looking for a career in. The certification course are career oriented and generally done by people who are working and need to enhance so at this point of time, I cant really advise on that.

Write to me specifying which state you are from and then we will look at options for you.