I've searched the docs but couldn't find it.

I want to change the name of the CMS field "Content" to something else and also change the HtmlEditorField to a textField.

Thanks

The easiest way to do it is in your Page class getCMSFields(), untested:

public function getCMSFields() {
$fields = parent::getCMSFields();
$fields->removeByName('Content'); // Might be 'Root.Content' or 'Root.Main.Content', not sure which
$fields->insertAfter(new TextField('Content', 'Your Field Name'), 'MenuTitle');
return $fields;
}

'Content' has a data type of 'HTMLText', but I think you should be okay to save standard text input into it. Not sure how to change its datatype if not.