first you have to enable “rewrite mod” which is apache server feature click here

then use the following lines in your .htaccess file
to remove WWW

RewriteEngine On
RewriteCond %{HTTP_HOST} ^www.example.com$ [NC]
RewriteRule ^(.*)$ http://example.com/$1 [R=301,L]

to force WWW

RewriteEngine on
RewriteCond %{HTTP_HOST} ^example.com$ [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]

Please, note that the .htaccess should be located in the web site main folder.

for farther reading about .htaccess Check this great post

sudo a2enmod rewrite

For Windows

1. Find the httpd.conf file (usually you will find it in a folder called conf, config or something along those lines)
2. Inside the httpd.conf file uncomment the line LoadModule rewrite_module modules/mod_rewrite.so
3. Also find the line ClearModuleList is uncommented then find and make sure that the line AddModule mod_rewrite.c is not commented out.

restart your apache service and you are cool to go