I cannot find any information about how - all I get is how to renew my existing subscription not how to change it.
There are 3-ways to do what you want to.
1- To upgrade an existing subscription to Pro Plus or Premium you should contact support.
They can upgrade your existing subscription without losing any time on it, and often at a special price.
To start the support process fill in this form: Support Contact Form.
Once you click ‘Next’ to submit the form you will be given a support ticket/ID number onscreen, with a summary of your request.
(If you don’t see the ‘Next’ button at the bottom left of the form then refresh the page before typing anything in).
The 'Live Chart dialogue should open automatically with a message from the AI Chatbot - if it doesn’t open itself then there is a ‘Start Chat’ button.
If the Chatbot can’t help then you will be passed to a real person.
IMPORTANT- Don’t just fill in the form and leave - If you do not engage with the chat then you will not get any reply.
2- You could cancel your current subscription renewal, wait for your current licence to expire, and then purchase a new Pro Plus (or Premium) subscription with a new licence key.
3- You could simply buy a completely separate CCleaner Android subscription from the Google Play Store. (It would be a separate subscription managed by Google).