You would have to change to a new crankset*, I would recommend a road triple for this (52/42/30), and acquire a road triple front derailleur that was suitable for a flat bar shifter.
Shimano makes the 2200 crankset which is quite affordable and will (I believe ... chainline might come into effect here) not necessitate a new bottom bracket. In my experience the FD-R443 derailleur (a road triple, for flat bar) works very well with this set up.
Can I now ask, why do you want a bigger chainring? My experience is people jump the gun on needing larger gears (unless your rides are almost entirely all downhill) and tend to push a much larger gear than necessary. I have found that alot of commuters and casual cyclists ride with a cadence of ~70 rpm. By making the assumption you are doing such, 15-25 rpm would show vast improvements in bike speed and would garner you greater cardio benefits. As well I would then assume, unless you are a highly trained athlete, that you will not spend a huge amount of time riding 48/11 @90 rpm.
Do you have a good set of clip in style pedals and shoes? This will help you work, in my experience, on your pedal stroke. By working first and foremost on your pedal stroke you will improve your muscle recruitment, improve your climbing (able to push the pedal over the top and scrape it across the bottom) and in general increase your average speeds.
Anyways, my two cents.
*When I say 'have to' I am referring to maintaining a semblance of smooth shifting. The reality is you can typically jury rig whatever you want and it often works. But it won't be smooth and it can be more of a pain in the arse than it is worth.