If you are the owner of a group conversation (e.g. the one who started the conversation), you can change the settings at any given time.
In the web interface go to ‘More’ at the top of your screen and click ‘Settings’. Change any setting you like.
On your mobile device you should open the group and in the menu on the top right pick ‘Options’. If you are the owner of the room, you will find the settings here.