Knowing exactly when to buy and to sell is the essence of trading, and it’s as much art as science. Generally speaking, it makes sense to look for markets that you think are mispricing the chance of an event taking place.
If you think the price is too low, buy ‘Yes’. If you think it’s too high, buy ‘No’. You could consider selling your shares if the price moves towards what you think is the right probability, or if events cause you to reconsider your judgment of the right price.
Whatever you do, just make sure to manage your risk so that if you turn out to be wrong about a few things, you don’t end up losing more than you feel you can afford.