when will we have to choose specialization in PUMBA MBA , whether it is in first year or in second year?

At the end of 2nd trimester i.e. in first year.

PUMBA follows Trisemester pattern that is we have three semester in one year. So in third trisem we have to specify the Specialization. That is we have to give the specialization at the end of first year.

In the first year , third trimester to be precise one has to choose specialization.