You might have to change icon theme/pack. There's tons on the KDE store and also probably packaged by your distro.

Personally, I am very much fond of the new icon. It's cute and adds a bit of character/whimsy. It's a breath of fresh air compared to all the flat monotonous design everywhere else these days

I'm not sure, if that will show up everywhere where the Dolphin icon is used, but you can change it in the menu and, I believe, the panel, by editing the Dolphin entry in the "Menu Editor" application. There's a button to select the icon there:

In that dialog, if you select "All" in the dropdown and then search for "stock_folder", that icon looks pretty close to the old Dolphin icon. Of course, if you've got the old Dolphin icon on disk somewhere, you can also select it with the "Browse..." button.