How can I change the notehead in rhythmic notation to get the same result as in the third bar of the attached file?
Unfortunately many for me important things are not possible in Dorico.
Have another go - you haven’t attached anything.
Typically noteheads can be switched by selecting the note(s), right-clicking, then going into the Noteheads section of the context menu and selecting the desired notehead.
You can alter the default (natural) noteheads for percussion instruments from the left panel of Setup mode.
If the noteheads you want don’t exist in Dorico, then (assuming you’re using Dorico Pro) you can Edit Notehead Sets from the Engrave menu (in Engrave mode) and customise to your heart’s content.
Without seeing the file, this is massive conjecture on my part - but just in case, here is some information about slash regions and slash voices in case that’s along the right lines.