Non Correct Version is Indexed

Hi everyone, I have a question. This is a handover client so I have not much historical data/knowledge about the website development. According to the client, the correct version of website is the one with www, for example, https://www.example.com. However, I found that there are almost 1/5 amount of pages indexed are from the non www, for example https://example.com. The client said that yes they have the non www before moving to the www version before, but it was 3 years ago and they already redirected it. However, as I said before, the non www version is still indexed, even years after it got 301 redirection. I also found Duplicate - Google Choose Different Canonical than Users issues in GSC, which is how I found out about this issue. My question is, what do I need to do about this? To my knowledge, I only need to redirect those non www to the correct version and the non www version will slowly be deindexed. But it has been 3 years (according to client) and the issues is in fact still appears in September (as far as GSC can show me) until today. Thank you in advance.