PHP update

Update of PHP doesn’t work on my site. I haven’t updated before because I have hardened PHP and it tells me I don’t need to. But now I cannot update wordpress or jetpack because I am using 5.4. I used PHP compatibility checker which said ok and so I changed to PHP version 7 on my c panel. Then I couldn’t log in to my site.
error-
Fatal error: Uncaught Error: Call to undefined function mysql_connect() in /home/granizcm/public_html/wp-includes/wp-db.php:1645 Stack trace: #0 /home/granizcm/public_html/wp-includes/wp-db.php(639): wpdb->db_connect() #1 /home/granizcm/public_html/wp-includes/load.php(435): wpdb->__construct(‘granizcm_wp569’, ‘S]h15@2SPw’, ‘granizcm_wp569’, ‘localhost’) #2 /home/granizcm/public_html/wp-settings.php(109): require_wp_db() #3 /home/granizcm/public_html/wp-config.php(89): require_once(‘/home/granizcm/…’) #4 /home/granizcm/public_html/wp-load.php(37): require_once(‘/home/granizcm/…’) #5 /home/granizcm/public_html/wp-admin/admin.php(34): require_once(‘/home/granizcm/…’) #6 /home/granizcm/public_html/wp-admin/index.php(10): require_once(‘/home/granizcm/…’) #7 {main} thrown in /home/granizcm/public_html/wp-includes/wp-db.php on line 1645
I have gone back to my original PHP and now my site works but that leaves me unable to update. I struggle with these technical issues and would appreciate advice. Thank you Cathy

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