If by "rounded" you mean stripped, then you can follow the next steps:
- Get a hold on a dremel with a cutting wheel, and Draw a line from side to side(Over the center, like a Diametre line) about the same depth of the hex tool that was supposed to loose the bolt.
- If possible, rotate 90 degrees and do the same a point 1
- Dip the the part in "Light Coke"(this thing works wonders on siezed bolts, not so much on your health tho) for about two hours. If you can do it overnight, that's ok too.
- Optional to (3.), pour some wd-40 on the bolts and threads. If it's has threadlocker then you will need a solvent, like Isopropil alcohol, acetone, or just a plain soldering iron to heat the bolt and soften the thing.
- Use a Slot ScrewDriver on the lines you made with your dremmel.
This is by far the safest way to remove stripped bolts.
Other method i've used include to drill a hole on the bolt, and then screw a special tool with counterclock thread to unscrew the bolt out of place, while you screw in the tool. Also you could just drill the bolt out, but you might get to damage the threads on your dereaullier.
The last method, only works on a couple of situations where the thread are not seized, is to take the head of the bolt out. This will take tre preasure off the thread and you can fiddle with it until unscrewd. Did this on a KTM Duke 390.