Bike light blink speed adjustment
The easiest way to do this in small quantity is to use a microcontroller, such as a Microchip PIC10F or 12F series. There will be only a few parts in the circuit and a page or two of code. The micro should be under a dollar in single quantities. 

The best way to do it in production quantity is a COB CMOS die designed for just that function, a microcontroller would be too expensive. 

It could also be done in small quantities with a counter-decoder chip (eg. 4017) and debounce circuit and a CMOS 555, but it would be substantially more complex (perhaps ~20 parts including 3 chips) than with the micro- so only for those suffering from extreme code aversion.