Easy----put hand in pocket; remove wallet; use contents to pay for upgrade!!!As D&B say, we are experienced cruisers and its the only way that has worked for us!!As an alternative, marry the CEO of a cruise company!!!
Phone the travel agent you booked with.
Are you prepared to pay the extra cost? Or are you after an upgrade without paying for it? If you are prepared to pay the extra contact the cruise agent that booked your cruise or contact the cruise line.
If you are after an upgrade without paying the extra, do not expect to much. I have seen many answers by very experianced cruisers (50 cruises and 40 years) on this website and the general answer is it is very rare to get a upgrade from a inner cabin to a balcony. Best of luck.