One word: DON'T!
AC and DC switches are constructed very differently. If your switch isn't rated for DC, it should never be used for DC.
All switches (and relay contacts) arc when opened. 3 things can break that arc: distance, an interrupting barrier and breaking the flow of current.
In an AC circuit, the current is broken many times each second and this is the primary method of breaking the arc in most AC switches. A tiny gap of less than a millimeter is usually all that is needed.
In a DC circuit the current is continuous and so you either need a much larger gap between the contacts or some sort of barrier to break the arc.
Watch this video:
Although the voltage is higher, the effect is the same. Notice how under AC the tiniest gap breaks the current path, but when using DC, it takes many times more gap to break the circuit.