We will not let your domain expire without giving you an opportunity to renew it. If you purchased a domain after 11 August 2011, you are opted-in for auto-renewal, so you don't have to worry about your domain expiring.
For more information about auto-renewal, take a look at this tutorial: What are the benefits of auto-renewal?
You can check your domain's auto-renewal date in My Yola > Domains - the auto-renewal date is displayed next to the domain name.
If you purchased your domain prior to 11 August 2011, you can renew your domain manually. Go to My Yola > Domains and click on the Renew button to renew your domain.
Currently you have the option to renew your domain for a number of years and in various available currencies. These prices were chosen based on a number of different factors and you will be able to use the currency of your choice upon checkout. The cost of renewing your domain will depend on the domain extension you bought. .com domains cost $11.95/year to renew.