why is SHA1 used? How do I get SHA256 to be used?

[Sam Smith]

Here's the result of ShowPRef for my key:
Cipher: AES256, AES192, AES, CAST5, 3DES
     Digest: SHA256, SHA1, SHA384, SHA512, SHA224
     Compression: ZLIB, BZIP2, ZIP, Uncompressed

SHA1 is showing up second. So when I sign a message, why isn't SHA256 used? The headers on my emails appear to show SHA1 as the hash being used.

I no longer consider SHA1 secure. Neither does the U.S. Government. So I don't want it to be the default hash being used.

How do I get SHA256 to be the default hash used when I sign emails and encrypt them?
 		 	   		  
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