you should got to your shop root Opencart directory via FTP or Cpanel ...
Open file .httaccess (if it is not exist you can rename htaccess.txt to .htaccess) and code below RewriteBase /
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ https://www.yourdomain.com/$1 [L,R=301]
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^(.*)$ https://www.yourdomain.com/$1 [L,R=301]
note: change yourdomain.com to Your domain
e.g: our domain mmosolution.com
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ https://www.mmosolution.com/$1 [L,R=301]
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^(.*)$ https://www.mmosolution.com/$1 [L,R=301]
Redirect to HTTP non-www to HTTPS www htaccess for Opencart