hi,

after completing diploma you may apply for 12th equivalent certificate to respective technical board of education from where you have completed your diploma courses. however you must have the following documents keep in handy with you:

• Certificates of all semesters of diploma (Original + Self Attested Xerox Copies )
• Final Board Degree Certificate of Diploma/ Provisional Certificate (Original + Self Attested Xerox Copies )
• Leaving Certificate from Collage (Original + Self Attested Xerox Copies )

I have attached a link which is available on official website of directorate of training and  technical education delhi.

http://tte.delhigovt.nic.in/wps/wcm/connect/DOIT_TT/training+and+technical+education/order+and+circulars/application+for+result+and+marksheet+and+other+queries

it would be best to contact with college authority first  and then proceed to apply.