how to charge a non-polar capacitor?
The difference between a polarized capacitor and a non-polarized cap is that the non-polarized capacitor doesn't care what the polarity of the applied DC voltage is.

A polarized capacitor can be charged with a DC source but you must ensure that the polarity is correct: plus to plus, minus to minus.

A non-polarized capacitor can be charged with a DC source without worrying about which lead is positive and which lead is negative.

If you take a close look at the capacitor, you will see that it is marked differently than a polarized capacitor in that there aren't any polarity markings.

One exception to this is old circular axial film or foil non-polarized capacitors: sometimes there is a band or mark at one end.  This band does NOT represent polarity.  Rather, it marks the lead of the capacitor that is connected to the foil that is the outer layer.  This is irrelevant if the capacitor is used as a coupling capacitor but can be useful if the cap is used as a bypass cap to ground.  Knowing which lead is the outer layer of foil allows you to connect the outer layer of foil to ground - this can reduce noise pick-up in sensitive circuits.