How can I switch from r8169 to r8168 on an installed system?

In a way of explanation:

Your question wasn’t specific to Desktop Environments > XFCE.

Since there is no “Networking” Category and

Welcome in the Newbie category

Hence the choice of category.

If you find another more appropriate section for your issue, ask any moderator to move the topic there.

Have a nice evening!

@pebcak