Coupler replacement on E-R Model Shark Nose Diesel

I want to replace the horn/hook couplers on some E-R Model Sharks that I have bought recently. I have one locomotive that came with knuckle couplers already. I have not been able to figure out how to remove the horn/hooks let alone replace them with knuckles. Anyone have any ideas?

I just looked at the Kadee coupler guide, replacement is possible on that model, but be careful, You may need to create a new coupler box if the existing one doesn’t give easily. Look in the Walthers catalogs under couplers and find the right set for your needs, #5 is universal , but may require something else. Either way, use caution, or just use that engine with other cars with x2f(hornhook) as in maybe a work train or something like it.
More later if I can find that book I need for those E R models,etc.
ken_ecr

See: http://www.kadee.com/conv/pdf/erf101.pdf

There is a pin in the center of the coupler box. Pull it straight up with a needle nose pliers. I replaced mine with a McHenry KS coupler, as it was simpler that going the Kadee route that require modification of the unit nose.

Thank you, gentlemen, for your replies. You have been most helpful. I think I got it figured out now.