Changing the frequency of a flashing LED
The LED can only be turned on when the BC557 transistor's base is connected to ground(it's a PNP transistor which is being used as a switch).

The base of that transistor itself,is being controlled by BC547(NPN transistor) which connects the base of the BC557 transistor to ground to light up the LED.Base of BC547 is being controlled by charging and discharging of the capacitor.I suggest you read about capacitor charging time which will explain the change in frequency due to change in capacitance or resistance.You can also look up LED flasher circuit explanation with different types of circuit design.