You have 2 options. Either exchange the berth during the journey or if berths are available go with fresh booking by giving the option, book only if lower berth is allotted.

Please specify exact train number, date & class then we can help you better.
Regards
HE2L

While travelling one of the options is to request some young man who has a lower berth to exchange his berth with the upper berth. Another option is to cancel the ticket and try afresh to get a lower berth in which case there is no guarantee that while booking next time a lower berth will be available as there is a good chance that all the lower berths are exhausted. One can also try to get a lower berth by booking one's ticket by logging into IRCTC website and at the time of booking exercising the option "Book my ticket only if at least one lower berth is allotted."

i had booked 3 tickets in the train no 11017 for the date of 5th feb 2012, which is 2 1/2months away, and i got ticket nos 2,4,6 in 2a which is all uB, Iam travelling with my mom, father(senior citizen) and my son 4 yrs old. as you had suggested if i cancel the earlier ticket, i lose money right???

As reservation for all the berths is conf, a cancellation charge of Rs.60 per ticket will be deducted and rest of the money will be refunded.

If you do not want to lose money then it is advisable to request fellow young and single passengers for exchange. Most of the time people will cooperate specially if you are travelling in 2A looking into age of your parents.