Use one of the following in .htaccess to rewrite the URL:

#Non-WWW to WWW version:
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^domain.com [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [L,R=301]

#WWW to non-WWW version:
Options +FollowSymlinks
RewriteEngine on
rewritecond %{http_host} ^www.domain.com [nc]
rewriterule ^(.*)$ http://domain.com/$1 [r=301,nc]

Please be aware that URLs are also set in scripts such as Wordpress or Joomla in their databases, admin sections, or config files which may need to be edited.