How do i change hostname displayed under 404 errors

I have configured a new server but the first domain i hosted on that sticked every where i just want to change the domain name displayed under 404 errors & directory listings like
Apache/1.3.34 Server at myfirstdomain.com Port 80

I make changes under etc/hosts but didnt helped in some domain its showing my root domain and somewhere its showing that first domain.

 

 

 

 

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