Replacing the standard sight is just a matter of carefully pushing out the pins, removing the sight, inserting the new sight and replacing the pins.
Make sure the sight is held tightly in place by the pins.
Something to consider: The Patridge target front sight is TALLER than the standard sight. The Target front was for use ONLY with the Colt Ellison target rear sight.
Chances are, the Python will shoot low and there MAY not be enough elevation available in the standard Colt Accro rear sight to get on target, without having the rear sight cranked way, way up.
However, the factory part number for the Patridge target sight is: #53960B
The Patridge sight is usually available from Gun Parts Corporation.
Their part number is: #926150
http://www.e-gunparts.com/
Recently, The Sportsman's Guide has been selling a two-pack of these.
SG #HX4A-71279
http://www.sportsmansguide.com/
NOTE: I've heard that some new Colt Python front sights are plastic.
I ASSUME the Sportsman's Guide sights are steel.