if you want to fix mistakes in your messages, click the More button on the bottom right corner of your message, click Edit, and do whatchu gotta do.
as for a sequel, it's unlikely it'll come out soon; if it'll come out at all. Slime Rancher will keep getting updated for a while. at least, until team Monomi thinks the game is "complete" (which it honestly isn't. very little content. you can get to the glass desert in 24 minutes or less, which is the end-game area. i'd say you could complete this game 100% in 4 days, if not less.)