I would check its documentation to see what is consider an acceptable range of temperature. I was really shocked when I found Macbook Pros can reach high temperatures safely, and so it's not worth messing around with your computer at all of it's within specs.
I play RS3 on my older laptop too (not an Apple product). I've had it since 2011 although it was already an older model manufactured in 2005. If yours is a notebook, then you can use a notebook cooler too! You'd be surprised how much that helps.
Other easy things might involve placement of your computer. Does it have good airflow, and is it not crowded in an enclosed area? You have to make sure it can dissipate that heat off to somewhere. If the flow is no good, then it might need a cleaning- you might be able to get away with a low cost compressed air / air duster.
Can you re-grease your CPU with thermal paste? It requires a bit more technical talent and I would save it for last; possibly take it to a specialist or a tech savvy friend.