Message expiration, I just cant figure how to use/change it?

Sorry i must be going straight mad, but I just can not find how to change or switch on/off the Message expiration. There is no dropdown option or anything like that that i can edit?

I have attached a shot of one my test conversations, it says
Current message expiration 4 weeks

Another of mine says Message expiration off

But i can’t change either of them and its driving me nuts that it should be so straight forward, why can’t I just see the answer?
Where do i set the Message expiration of a conversation?

Plus , once i have had this answered, my next question is why does the conversation that should have a Message expiration of 4 weeks have a history of many many months?

I am using firefox on my Linux client, Nextcloud is running on my truenas server. Everything is up to date.

I have found this info 5 features in Nextcloud Talk you didn't know about

Its pretty clear that i should be able to change the expiration, but i can not, there is no dropdown menu in any of my messaging (talk) accounts anywhere. I have searched and searched?