You can change the signature or name that appears on messages you send by following these steps.
Web:
Step 1: Click your name in the bottom left hand corner of the screen.
Step 2: Select 'Settings'
Step 3: Select 'Personal Info'
Step 4: Type in your preferred signature and save.
Mobile:
Step 1: Select the 'Profile' tab at the bottom of the screen.
Step 2: Click the pencil icon in the top right corner of the screen.
Step 3: Type in the signature you would like to use and click 'Save' at the top.