How can I patch qmail-smtpd-auth?

[bc201]

I have get qmail-smtpd-auth-0.26.tar.gz.
 "tar xvfz qmail-smtpd-auth-0.26.tar.gz"
Then I get qmail-smtpd.patch 
Now I do not know how to use is.
Please tell me how to do it. Thanks.
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