I have 20x2.4 tyres and need to fit it on 20x1.95 rims but I don't know if I can fit it on is it possible to do this
You should be able to fit it as the tire width is wider then the rim width BUT you should check if the tire will fit in your frame.
Almost 2" wide rims sounds huge, but I guess wide 20" BMX rims exist.
Presumably you measured the outside width of the rims (rim width is usually specified by the inside width where the tire beads sit).
As the tire nominal width is greater than the outside rim width, the tires should be OK.