The side-mounted Kalart rangefinder on my Century Graphic is fitted with a telescope. This is dirty inside. Does anyone know how to get it open for cleaning?

Each lens is held in place by a spring wire ring. You should be able to catch the ring and pull it out and then the lenses will come out and you can clean them

The eyepiece extension screws into the rear view window.
The windows screw into the rangefinder body.
https://graflex.org/speed-graphic/kalart-manual.html
https://graflex.org/helpboard/viewtopic.php?t=6684

Does your rangefinder actually have a telescope, with lenses in it? I ask because I have a Miniature Speed which has a simple tube with an eyepiece attached to the Kalart. There are no lenses in it and no mounts for lenses - it simply serves as a guide for the eye. The eyepiece screws into a mounting bracket on the side of the camera near the back, and the connecting tube is just loosely held between it and the rangefinder window.