Older versions are available by choosing "Learn More", which appears when you hover to the left of the open button for Premiere. From there, choose the 3 dots beside the Open button, and you should be given options for previous versions,
You might want to uninstall all the other versions first.
Please go to Adobe Customer Service (Make sure to sign in to your Adobe account first and allow popups and cookies on your browser. You may not be able to see the chat window if using a VPN.): https://helpx.adobe.com/contact.html?rghtup=autoOpen will automatically open the chat window. Type "Agent" in the chat box to bypass the chatbot and be connected to a person.