but what is the process to apply for both. 

Suppose I have got admission in one of the colleges through merit based procedure in 1st cutoff and I have also cleared the cutoff of entrance test. Then, to get admission through entrance based procedure, do I have to cancel my previous merit based admission or I can continue with that and take admission in new college ( entrance based) and decide after all the cutoffs which one to choose ultimately.

See, if you will be getting merit-based admission in the first cutoff in the course of your choice, then go for it, book the seat. As it is most important in the DU admission process. Now the DU JAt result will be declared almost after the 4th cutoff, assuming that you will get BMS, BBE OR BBA in the college of your choice, You have to first cancel your admission in the previous college which you got through your merit and go for the first college of which you meet the entrance criteria. If you miss out and wait for all cutoff, your seat will be gone and technically you won't be getting admission.I hope this helps.

Thank you nishtha for the help.