Increasing a pattern

How can I increase my pattern from a size 9 months to a 12 month size/

Welcome to KnittingHelp!
One of the easy ways to do this is to use a thicker yarn and larger needles than the pattern calls for. If you knit a swatch with your yarn and needles, you’ll have an idea of how much larger the resulting project will be.
Can you tell us the name of the pattern or give us a link to it?