Changing the handlebars on your bicycle can lend an entirely different feel to your ride. Drop bars are designed for the more serious cyclist, because they allow you to lean forward in an uncomfortable but aerodynamic "tuck" position. Straight handlebars, on the other hand, are typically found on commuter or casual bikes where comfort is more important than aerodynamics.
Remove the bar tape from the drop bars. You may need to flip back the rubber brake hoods to access the tape that wraps underneath them.
Release the brake cables. Use wire cutters to snip the small metal crimp off the end of each cable at the point where it feeds out of the brake. Loosen the bolt on the brake with a hex wrench. This allows you to pull the cable through its housing and out the end of the brake levers (the part that is mounted in the handlebar). You will need to pull the brake levers in toward the bike to expose the end of the cable as you feed it out.
Release the shifter cables in the same manner if you have a newer bicycle with indexed shifting (i.e., you shift via the brake levers rather than with levers mounted on your downtube). Again, cut the crimp off the end of the shifter cable, loosen the bolt and slide the cable out through its housing.
Loosen the clamps holding the brake levers in place by unscrewing the hex bolts. Gently slide each of the brake levers off the handlebars.
Unscrew the hex bolts on the stem of the bicycle that hold the handlebars in place. Manoeuvre the drop bars through the stem and off the bike.
Slide the flat handlebars through the stem clamp and tighten the bolts, making sure that the handlebars are set in place evenly.
Install brake levers and/or shifters. If your bike had indexed shifting on the drop bars, you will need a new set of brakes and a separate set of shifters. If your shifters are mounted on your downtube, you can reuse the brakes from the drop bars on the flat handlebar by rotating each one 90 degrees.
Put grips on your new handlebars. To make it easier to slide rubber over the metal, lubricate the inside of the grips with water. The water will evaporate and leave your grips firmly in place.
Reconnect brake (and, if necessary, shifter) cables. As explained by BicycleTutor.com, you feed the cable in reverse through the housing and the brake bolts. Adjust brakes so that the brake pads are almost touching the rim of the wheel, then cut any excess cable and put a crimp on the end.
Adjusting brake and shifter cables can be frustrating. If your bike does not have indexed shifting, save yourself some trouble by leaving the brake cables intact. You can slide the entire brake lever off and back onto the new handlebars with the cable still connected.
If any of your brake or shifter cables are frayed, discard and replace them.