I have a photo of an object on a black background, and during the editing process the cmyk values of the black background are c=75 m=68 y=67 k=90. As this image is to be placed on a business card, I was wondering if it is possible to change the values to 100% K or 40% C 100 %.
I'm using CS5 creative suite.
Just clip the object (with a Layer Mask for example or if it is a technical object and in focus with a Vector Mask) and put in on the proper background in Photoshop or a layout application.
May I ask why you work in CMYK?
When editing photographic images separation is usually best one of the last steps or the last step (and performed on either a copy or in the layout application).