Android
Unread
- Go to the chat list.
- Tap and hold on a chat.
- Tap to select any other chats.
- Tap Menu at the top.
- Select Mark as unread.
Read
- Go to the chat list.
- Tap and hold on a chat.
- Tap Menu at the top.
- Select Mark as read.
iOS
Unread
- Open Signal to view your chat list.
- Tap and hold on the chat and swipe to the right.
- Tap Unread.
Read
- Open Signal to view your chat list.
- Tap and hold on the chat and swipe to the right.
- Tap Read.
Desktop
Unread
Select the chat > Chat Settings 
> Mark as unread
Read
- Leave the chat.
- Select the chat.