The easiest and cheapest thing to do is swap out your rear cassette. You don't mention which components you have nor what your budget is, but you can probably find a new 11 (or 12)-28 cassette (I know SRAM makes 'em) for $50 or so depending on which model you get.
Installing is a piece of cake if you have the cassette tool & a chain whip and worth doing yourself. You may need to adjust the B screw on your rear derailleur when adding a larger cassette. Check youtube or parktooks.com for how-to videos.
You can probably get to a 16 tooth spread between your two front chain rings without impacting shift performance too badly but really, what's the point of going from 53 to 55? If you're spinning out a 53/11 combo you probably just need to get more aero on those big descents.