Either way actually works, it depends on how much bigger you need to make it and if you like the resulting fabric. Check out this article for
Knitting a Swatch it hopefully makes sense about what will happen when you change needle size.
This article is about
Reading a Pattern and will help through some of the common issues.
And finally, this article discusses
Adjusting a Pattern I hope all this helps, if it doesn't, just come on back!