I install the update of Lightroom (7.3) and the black&white doesn't works. Can you help me?
You need to explain what are the steps you use, and what happens, in detail. Lots and lots of detail. You can't simply say "doesn't work" for us to know what is happening.
I suspect you are referring to the fact that it is not in the place it was before. The B/W options appear only if you click B/W in the Basic panel.
it was like this
B&W only shows up now when you click Black& White. in the Basic Panel
Retrieving data ...