You did your diploma in mechanical engineering and presently working as a Digital Marketing & Tech Manager in a private firm. You want to continue your career in the same line only and there is no idea of going back to your core subject which is mechanical engineering. In such a case, there is no use in doing BE/ BTech in Mechanical Engineering. I feel you should improve your skills in the area you are working.
I suggest you go for improving your qualification in the digital marketing field. There are institutes offering degree courses at UG as well as PG levels in Digital Marketing.
The following are the courses that are available
1. BBA in Digital Marketing
2. MBA in Digital Marketing
3. Online Digital Marketing
4. Digital Marketing Certificate courses
you can think of BBA through distance mode or online. Once you complete your BBA, you can think of doing MBA so that you will have a chance to have a better career. Massive Open Online Courses (MOOCs) offering platforms are there.
upgrade, Simplilearn, DigitalVidya, Udemy, and Coursera, are some such MOOC platforms where you can find a number of online courses which will be useful to you for your career progress. you can also find the courses offered at the Government of India's MOOC platform, SWAYAM, in this field