Hello
The mixers have the double "chorito", that is from above and below, some are screwed, others with screw. You should carry the stem that you removed so that they give you the one that corresponds.
In your query you said that you disassembled it above and found a screw. I leave you an image of a type bobbin so you can take it apart. Normally the rod only rotates, it cannot be raised or lowered, apparently the body thread would be worn, in that case the entire head must be changed.

You have to remove the packing nut so that everything comes out and I clarify this is a type image that may not have to do with your mixer. But basically it's the same, I hope this helps you disarm
Regards