You don't want to blow compressed air deeper into crevices (don't stick it to the outside vents of the laptop and blow inward - not a good idea ).
You need to take apart the laptop carefully (download the .pdf manual for your laptop, they have detailed instructions for repair and for replacement part numbers). Each laptop is different. Desktops are easier to clean.

I've taken apart my parents' laptops to fix stuff from time to time, but not being an expert and because everything is crammed in so tightly, I've accidentally damaged them in minor ways (down arrow key no longer works on one of them. The videocamera doesn't work on the other. Neither feature was used much anyway).

Sometimes videocard overheating occurs for reasons other than dust - my videocard (in a desktop) had a flaw in it that makes the oil evaporate from around the fan, so the fan slows down and can't pull the heat off fast enough - I have to re-oil it about every six months (and I dust out the desktop at the same time, while I'm in there). That's rarer though.

Desktops are a whole lot easier, since there is plenty of room to work, and desktop cases are easy to open.

With both my mom and dad's laptops, when I want to get inside the case, I have to take off the keyboard and the entire front face the keyboard is in, disconnecting the keyboard and touchpad and such - it's a multi-step almost hour-long process to take it apart and put it back together. With my desktop, I basically click a button, and one wall of the case slides off, and I have access to the entire thing - a 5 minute clean job, 15 if I'm oiling the videocard.

Make sure you have tiny screwdrivers to unscrew the screws - don't try to cram large screwdrivers into small screwheads. You're looking for ones larger than eyeglass screwdrivers, but smaller than construction screwdrivers - they aren't expensive and you're local Ace Hardware or Lowes will probably have a set.

If you plan to do this regularly, see if you can get a deal on compressed air. I bought a 6-pack off Amazon for $23, so it worked out to about $4 a can. Each can has lasted awhile - Amazon said I ordered it two years ago, and I'm on my 4th of the 6th cans (one can was defective and I threw it out, but it was still a good deal).

I was nervous the first couple times I did so (and still am a little when it comes to laptops), but hey, if something breaks, that's the cost of education - fear of damaging something shouldn't discourage you from attempting to do so, and you'll learn and gain practical skills through attempting to do what you've never done before.

Both of my parents' laptops overheat when watching netflix for too long, because they are cheaper laptops (<$500) and are in use throughout the day with little rest for them.

There's little that can be done to 'fix' that, except to notice when it's heating up too much and close the lid of the laptop and flip the thing upside down to let it cool off. You could also get a good cooling pad to help vent the laptop to pick up the slack of a weaker fan - we've done that about three times, but the cooling pads all broke after a month or two. I think they were cheaper $10-$15 bargain ones, so maybe the more expensive ones last longer.