upgrade from 151 to153

Tried to do simple upgrade copy & screwed it up! Admin loading response:
"Call to undefined function: mysqli_connect(). Please install the MySQL Connector for PHP"
Checked php & sql versions of my hosting & their in range.
Appreciate everyone's/anyone's help, I'm not very "techie" at my age!

rehopper:

Tried to do simple upgrade copy & screwed it up! Admin loading response:
"Call to undefined function: mysqli_connect(). Please install the MySQL Connector for PHP"
Checked php & sql versions of my hosting & their in range.
Appreciate everyone's/anyone's help, I'm not very "techie" at my age!

Need more information. What is a "simple upgrade"? What process was followed to get what you have? What was used to put the files on the server? Where did the copy of 1.5.3 come from, anything else in the posting guidelines that might help in this situation?

rehopper:

Tried to do simple upgrade copy & screwed it up! Admin loading response:
"Call to undefined function: mysqli_connect(). Please install the MySQL Connector for PHP"
Checked php & sql versions of my hosting & their in range.
Appreciate everyone's/anyone's help, I'm not very "techie" at my age!
It sounds like your host might not have enabled the mysqli interface, which is required for Zen Cart v1.5.3 due to mysql_* functions being deprecated (i.e. no longer included, removed from) PHP versions 5.5 and later.

You should contact your host and ask them to enable mysqli support for your installation.