The TaxJar Magento extension uses cron when available to refresh tax rates in your Magento installation once a month. Sometimes it is necessary to force a refresh when you change your nexus configuration or when your server does not support cron scheduler.
To refresh simply make sure your TaxJar account has all the proper US states and addresses. Then inside of your Magento installation navigate to the System->Configuration menu and find the TaxJar panel. Once their confirm your API Token and click Save Config.
Your Magento sales tax rates should now be refreshed for your specific states.