Hi and welcome to the forum and PHP programming

The thermometer.php script has several settings inside the script
//------------ settings ------------------
$wxSoftware = 'WD'; // set to 'VWS' to use VWS WeatherFlash files
//
$UOM = 'F'; // set to 'C' for Celsius/Centigrade, 'F'=Fahrenheit
//
$autoScale = true; // set to false to disable autoscale.
//
// you only have to set one of these correctly based on the $useWD selection
// $wxSoftware = 'WD' : set the $clientrawfile
// $wxSoftware = 'VWS': set the $wflashDir
//
$clientrawfile = './clientraw.txt'; // relative file address for WD clientraw.txt
$wflashDir = './wflash/Data/'; // directory for the the VWS wflash.txt and wflash2.txt files
// // relative to directory location of this script (include
// // trailing '/' in the specification
//
Based on your questions, I'm assuming your using Weather-Display (clientraw.txt) and want the temperature displayed in Celsius/Centigrade (range 55 to 5).
Is your Weather-Display uploading clientraw.txt to the same directory as the thermometer.php script? If so, no change is needed to the script. If not, then change $clientrawfile = './clientraw.txt'; to point to the relative file address of your clientraw.txt file. It would help if you would put your website address in your profile.
To change to Celsius/Centigrade, just change $UOM = 'F'; to $UOM = 'C;
To change the default range from 40 to -5C to 55 to 5C, just change
// Centigrade settings
$TmaxC = 40; // maximum °C temperature on thermometer
$TminC = -10; // minimum °C temperature on thermometer
to
// Centigrade settings
$TmaxC = 55; // maximum °C temperature on thermometer
$TminC = 5; // minimum °C temperature on thermometer
Hope this helps..
Best regards,
Ken