Control Panel > Content Rebuilder > Transition Actions
The UTF8mb4 Transitioning Tool is located within the bottom section, just under the HTTP to HTTPS Transition Tool.
Update All Tables to UTF8mb4 Update database table collations to 4-Byte UTF-8 unicode language encoding (eg. utf8mb4_general_ci). 4-Byte UTF-8 Support requires MySQL 5.5.3 or above. Depending on the size of your forum and tables, this process may take a long time to complete. Note: It is highly recommend to create a database backup as this process cannot be undone.