You can rename new or existing conversations on any device by looking at the group’s unique information page.
To get there from the desktop application, click the information icon (the letter ‘i’ with a circle around it) in the top right corner. Click the current group name, and replace it with your desired name, then click done to save your changes:
For the mobile applications, simply tap the group name at the top of the screen when you are in the conversation. This will pull up the group’s information page, and by tapping on the blue “Edit Name” button, you can change the name of the group.