Question: how to change a default signature in outlook?
Answer: In Outlook, select “File“ > “Options“.
Select “Mail” on the left, then click “Signatures…”
Select “New“.
Give the signature a name. ... 
Under the “Choose default signature”area, select the “E-mail account” you wish to apply the signature to. ... 
Click “OK“, then “OK” again and you're done.