Sorry to say this, but you're using Gentoo and asking basic questions like this?
To answer you: For changing your icons, you can do that in control center, appearance and themes.
For changing the K-menu (the K icon) you need to replace the .png, as there's no tool to do that directly.
From
http://docs.kde.org:
5.2.
Is it possible to change the K button of the panel to another picture?
The simplest way is to download a new icon theme with a K menu icon you prefer from
http://www.kde-look.org and install it using the KDE Control Center.
In order to change only the K menu icon you can overwrite the kmenu.png image for every size in an icon theme set. So, for KDE default icons this would be $KDEDIR/share/icons/default.kde/size/apps/kmenu.png, where size is one of the sizes included in the KDE icon theme.