For search engine optimization, you will need to choose only one version of your domain name to prevent duplicate content. That could be either www.example.com or the non-www version example.com
To redirect one to other, you will need to make sure Mod_Rewrite is enabled on your server. Edit your .htaccess file and add the code below. Note that in this case, the non-www version is redirected to the www version and you will need to replace your domain name below.
RewriteEngine On
RewriteCond %{HTTP_HOST} ^example\.com$ [NC]
RewriteRule ^(.*)$ https://www.example.com/$1 [R=301,L]
If you already have a .htaccess file with RewriteEngine On copy only the following code after that line.
RewriteCond %{HTTP_HOST} ^example\.com$ [NC]
RewriteRule ^(.*)$ https://www.example.com/$1 [R=301,L]