How to modify a router's antenna?
You can replace the antenna with one that has higher gain , there are several types but in order to get high gain the antennas will have a pretty narrow beam pattern (will be directional).

A simple way to get some additional gain without changing your antenna is to use a reflector, for example http://www.binarywolf.com/249/diy-parabolic-reflector.htm

The reflector I talk about looks like