You get faster by riding faster.
Seriously, you have to get your body to adjust to a faster pace. To help that process, you can do "intervals". Here's some
tips.
There's a lot of mumbo jumbo in there. To simplify it, go the section where they talk about telephone poles. The idea is to go really hard between two telephone poles, then go at a more relaxed (but not turtle slow) pace for the next 3 or 4 poles. Do several of these during a given ride, then don't do it again for the next ride or two. You only need a couple of these sessions a week and you need to have adequate recovery time between them.
Also make sure you warm up a good 15 to 20 minutes before you start them.