On iOS, go to the Settings app, tap Mail, Contacts Calendars, scroll down, then tap Signature. Once there, you can change your email signature, and choose whether you want a different signature for every email account you have (tap Per Account) or One Signature to Rule Them All (All Accounts). In my case, I modified the stock iPhoneiPhone email signature to serve as a warning about the inevitable typos. Alles zu iPhone auf CIO.de
Once you're done, tap the back button or close out of the Settings app.
Android typically comes with two mail apps: a general email app that will work with lots of different mail services (aptly named "Email") and one for GoogleGoogle's own Gmail service. The two apps are very similar to one another, and the process of changing your email signature is the same for both apps. Alles zu Google auf CIO.de