Start with the gentlest approach: a water soak with a little Photo-Flo may work, and one answer suggests soaking for several hours if needed. If that doesn’t release the film, a small amount of mild dish soap in the soak was also suggested.

If the film is still stuck, another careful option is to sacrifice the sleeve: cut the sleeve apart and peel away the plastic layer by layer, using a very sharp blade only where there is clear space between the film and the sleeve. Test this first on a less important strip.

Kodachrome was sometimes finished with a thin lacquer coating, and if that is what has adhered to the sleeve, water may not be enough. In that case, one answer recommends trying film cleaner before moving to stronger solvents. Strong solvents such as lacquer thinner are much riskier and should be a last resort.

Practical advice: begin with the mild soak, handle the film as little as possible, and test any method on your least valuable negatives first.