To change your auto email signature in Outlook 2013/16, please follow steps below:

• From the ribbon bar, choose File -> Options.
• From the left panel, choose Mail.
• Then click the Signatures... button.
  

To create signature:

• Click the New button.
• Provide a name for your signature then click OK
• Type your signatures in the Edit Signature text box.
• When done, click OK.

Set Auto Signature:

• On the right hand side, under the session Choose default signature, set the signature you preferred for New messages: and Replies/forwards:
  
• Then click the OK button.
• Now when you create a new email, replies or forward a email the signature you selected will be added in the email.

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To change your auto email signature in Outlook 2010, please follow steps below:

• From the ribbon bar, choose File -> Options.
• From the left panel, choose Mail.
• Then click the Signatures... button.
  

To create signature:

• Click the New button.
• Provide a name for your signature then click OK
• Type your signatures in the Edit Signature text box.
• When done, click OK.

Set Auto Signature:

• On the right hand side, under the session Choose default signature, set the signature you preferred for New messages: and Replies/forwards:
  
• Then click the OK button.
• Now when you create a new email, replies or forward a email the signature you selected will be added in the email.

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To change your auto email signature in Office 365 webmail, please follow steps below:

• Login Office 365 portal.
• Go to Settings > Mail.
  
• From the left hand side, choose Mail -> Email Signature.
  

To create signature:

• Type your signatures in the text box.
• When done, click Save.

Set Auto Signature:

• On the right hand side, tick the options Automatically include my signature on new messages I compose and/or Automatically include my signature on messages I forward or reply to
• Then click the Save.
• Now when you create a new email, replies or forward a email the signature you selected will be added in the email.