If you are already a Yesware user, simply go to app.yesware.com/account/plan to see your current plan and to choose a new one.
Yesware offers four plans: Pro, Premium, Enterprise and Enterprise + Phone. You can find costs and feature descriptions on your plan page as well as online here.
Please note that the only plans that have a monthly billing cycle option are the Pro and Premium plan. The Enterprise and Enterprise + Phone plans must be paid upfront on an annual billing cycle.