Hi christian,
If you mean how do I execute atmel's default boot loader, It's too simple, just momentarily ground PSEN and then apply a Vcc pulse to reset, after reset reached ground state, release (float) PSEN, the atmel’s boot loader is at your service now. In fact it is so simple that I wish it was a bit more complex because if during reset a glitch or noise emulates PSEN attachment to ground, then chip behaves unexpectedly.