Can you change or swap out the Pokemon in a gym on Pokemon Go?

The other day I put a Pokemon in a gym and was actually able to train him up a bit more while he was in the gym. Today I can not do anything with him. I was hoping to switch him out with another Pokemon.

Is there a way we can switch which Pokemon are in the gym?

You can't swap a Pokémon with one of your own. You can however, under some conditions, swap your Pokémon with one at a friendly Gym. This is only possible when:

• The Gym is at the maximum level of 10
• All places in the Gym are occupied (10 Pokémon)
• You don't already have a Pokémon at the Gym
• The Pokémon you want to add has a higher CP value than the lowest

In that case, the Pokémon with the lowest CP value will be kicked out of the Gym and is returned to its owner. Source (pg. 6).

You cannot. All Pokemon are stuck in the respected gym until someone in an opposing faction comes and either knocks your pokemon out(if its a lower tier of the gym) or captures the gym outright.