Open your iPhone > tap the Setting icon.
Select Mail > Scroll down to the Signature option and tap that.
Scroll down to the Signature option and tap that Choose whether to create a signature for a Specific Account or for All Accounts (this option will only be presented to you if you have more than one account.
Tap the text field and delete the “Sent from my iPhone” placeholder line.
Then type in the information you want to be included in your signature or paste in your signature HTML.
Close settings > Open your mail app > Open a new email message to see your new signature there.