How to update a database entry with a wordpress plugin?

$newaddress = array( 'address' => '$newaddress' ); // Incorrect

Is not the same as:

$newaddress = array( 'address' => $newaddress ); // Correct

And you have a missing ; at the end of:

$myrows = $wpdb->get_results( "SELECT address FROM wp_mediopay WHERE id = 1" )

And before the above line you must also have:

global $wpdb;