Fatal error: Uncaught Error: Call to undefined method ModelUpgrade::mysqli() during upgrade

Connection successful
Fatal error: Uncaught Error: Call to undefined method ModelUpgrade::mysqli() in C:\xampp\htdocs\Books OpenCart Template\theme338\install\controller\upgrade.php:45 Stack trace: #0 C:\xampp\htdocs\Books OpenCart Template\theme338\system\engine\front.php(42): ControllerUpgrade->upgrade() #1 C:\xampp\htdocs\Books OpenCart Template\theme338\system\engine\front.php(29): Front->execute(Object(Action)) #2 C:\xampp\htdocs\Books OpenCart Template\theme338\install\index.php(82): Front->dispatch(Object(Action), Object(Action)) #3 {main} thrown in C:\xampp\htdocs\Books OpenCart Template\theme338\install\controller\upgrade.php on line 45 :

From Which version you are trying to upgrade?

There is no such file in OpenCart so you will probably need to contact the theme developer.