I think it should depend on whether the goggles you got have the flexible plastic/rubber around the lens.If they do, just bend and twist a bit and the edge of the lens will show. Put a fingernail under and work around to pull it out of the groove.Put the new one in the same way - fit into the groove and work your way around. The last little bit you flex the plastic back and push the lens into the groove. If you can't manage by yourself ,take them back to the place where you bought them to see if they can help .
Well, generally speaking, you should not replace your glass lenses by yourself. And in my opinion, it can be dangerous for you to replace it, for it can just harm your glasses. For example, they are specially designed, so when you do not replace lenses in a right way, it can just be hard to reverse it back. Also, it is possible for you to get scratches on the lenses. Also, some dirty objects can be led on your lenses. Anyway, you are not supposed to replace them by yourself.
You should firstly warm the snowboard goggles which will make it easy for you to get the lenses off the goggles. Then you could get the old lenses off with suitable strength. At last, you just change the new lenses in and make the surrounding line adjustable to the snowboard goggles. The process is so easy that you could have a try.