Suppose you put a battery across a resistor. Whichever way you connect it, mark the resistor + and - according to the voltage polarity marked on the battery.
Now reverse the battery. The terminal marked + is now negative relative to the other terminal.
This is all that is happening with an AC voltage, but it happens in a smooth sinewave manner. If the AC input was slow enough, you could watch a voltmeter measuring it reverse polarity. It would read positive voltage for half the cycle and negative voltage for the other half of the cycle.
The electrons actually flow very slowly, so that the electrons that flow out of the resistor are not the ones that flowed into it, but the current effect happens very quickly, at speeds near the speed of light.
When you reverse the polarity of the supply, either with a battery or with an AC supply, the electrons flow in the opposite way to the way they were flowing before. They always flow from the more negative side of the supply to the more positive side.