This should be an old-fashioned flush valve in your house, which is rarely used now. As shown in the figure

Disassembly is actually very simple. Please do the following:

First, drain the water.

Clean the water in the sink.

Second, remove the main body.

Because I don't know what kind of structure is at the bottom of your drain valve, I'll tell you separately.

The base shown in the above picture needs to be removed with a screwdriver, and it can be removed by turning the screw in the right half. Be careful not to drop the attachment at the bottom.

Just screw it directly like the bolt in the picture above.

You only need these two steps, because this is an old-fashioned split drain valve, so there are not so many accessories. Just buy one of the same size online and replace it. How to disassemble is very simple.