Not Redirecting from non www to www through .htaccess

[Mohsin59]

Trying to redirect my one of the domain to www from non www. Already try to update .htacess rewrite code but its not working.

Default .htaccess code:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

New Code which i have updated:

RewriteEngine on
rewritecond %{http_host} ^coolexample.com [nc]
rewriterule ^(.*)$ http://www.coolexample.com/$1 [r=301,nc]

so what is the solution and which code i write to redirect my site with www.

Thanks in advance