Redirect to www & vise versa (htaccess)

Edit your .htaccess place this code after RewriteEngine On
This lines makes redirection to www

# Redirect if NOT www.example.com (exactly) to www.example.com
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^www\.example\.com
RewriteRule (.*) http://www.example.com/$1 [R=301,L]

or

# Redirect if example.com (case-insensitive) to www.example.com
RewriteCond %{HTTP_HOST} ^example\.com [NC]
RewriteRule (.*) http://www.example.com/$1 [R=301,L]

Use non-www redirect

# Redirect if NOT example.com (exactly) to example.com
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^example\.com
RewriteRule (.*) http://example.com/$1 [R=301,L]

or

# Redirect if www.example.com (case-insensitive) to example.com
RewriteCond %{HTTP_HOST} ^www\.example\.com [NC]
RewriteRule (.*) http://example.com/$1 [R=301,L]