Changing your Emoji on slack, an ABC guild.

When you are logged in into your slack profile

Click on the three dots on the right side of your phone screen or profile and click on edit your status. 

Under "edit your status" click on the emoji face and when it appears at the top, click on it again and a search windows will appear, type in either "one" or "two" "depending on the emoji you want to use" bunch of emoji's will appears select your prefered emoji.

Below the emoji remove the "clear after" by clicking on it and select "don't clear" and save. You are good to go.