just installed slackware64-live-plasma5-current-04-23-2018 in virtual box.

Installed Slackware has crippled version of Oracle Virtualbox added. This is nice however it does contain only vboxvideo modules.

I would like to upgrade to the full version of virtualbox.

How can I remove existing version of virtualbox?

how did you install it, slackbuilds or via their sight instructions?
I m not sure how a slacbuild installs it, so kernel headers and such need to be taken into consideration.

slackware current ships only the xf86-video-vboxvideo package, that contains the video driver to use X in your virtual box virtual machines (not the host running the virtualizer).
if I understand correctly, you want to install the virtualbox guest additions to enhance your virtual machines: you can do it installing two packages from SBo in their modified versions for current, virtualbox-kernel-addons and virtualbox-addons.
download all the files from the links in two different folders and then run the SlackBuilds: if you don't know how to use a SlackBuild, have a look at the howto.

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If you have their installation package you can go, as root, to /opt/VirtualBox
and run
sh uninstall.sh
and that will remove virtualbox.

If you don't want to build the packages you can download everything you need from here,

https://www.virtualbox.org/

Work your way to the packages for Linux distributions. Go to "all distributions" and right click on "amd64" (or i386) and "save link as..."
Download load the extension pack.
Go to wherever you saved the packages on your hard disk and run
sh ./virtualboxwhateverversion.run
Once done, start virtualbox, go to preference and install the extension pack. Once you have installed a "machine," install the guest additions CD image.
Be sure to add a vboxusers group and your user name to that group.

My Slackware Live Plasma5 does not contain VirtualBox.
But Slackware's X.Org does contain a xf86-video-vboxvideo package. It contains the VirtualBox virtual video card driver. Perhaps you confuse this X.Org driver with the full VirtualBox, but in that case you are mistaken.