2.3.0 makes the currency switcher usable from the keyboard, hides the converted prices whenever a product's own price is hidden, and loads less on pages that don't need the switcher. It also raises the WooCommerce minimum to 9.1, so check your WooCommerce version before you update.
2.2.1 fixes two things you could see. On a Turkish site the plugin appeared in English, and the two cache-compatibility warnings kept clearing and coming back. Both are fixed, and there is nothing to do after you update.
2.2.0 is a privacy fix. On a store without a local MaxMind database, geolocation could send every new visitor's IP address to a remote geolocation service. It no longer can. Geolocation is also switched off by default on new installs. If you rely on it, read the last section: on some stores it now needs one setting to keep working.
MHM Currency Switcher is now listed on WordPress.org, and 2.1.0 is the version it arrived with — WordPress offers its updates like any other plugin's from here on. The release also finishes a naming clean-up that changes three things custom code can rely on, so if you have written CSS, JavaScript or PHP against the plugin, read the first section before you update.