I recently installed Fedora 23.
It uses Yum Extender to update, add and remove packages.

When running Yum Extender, most packages update with no problem.
However, I get the following error:

Dependency Resolution Errors:

Package: NetworkManager-l2tp-gnome-1.0.2-1.fc23.i686 (updates)
Requires: NetworkManager-l2tp = 1.0.2-1.fc23
Removing: NetworkManager-l2tp-1.0.2-1.fc23.i686 (installed)
NetworkManager-l2tp = 1.0.2-1.fc23
Obsoleted By: NetworkManager-l2tp-gnome-1.0.2-1.fc23.i686 (updates)
Not found
Available: NetworkManager-l2tp-0.9.8.7-4.fc23.i686 (fedora)
NetworkManager-l2tp = 0.9.8.7-4.fc23

What must I do to resolve it?

Are you protecting the base install? Looks like you have a repo that doesn't mix well with the base repo and since the extra repo has a newer version it is trying to install that but looking to the base repo for files it needs.

What do you have for repos installed?

seeing as fedora 23 is using DNF now

i would expect that you would have a problem with the OLD yum

I do not know what you mean by "protecting the base install".
I recently (a couple weeks ago) installed Fedora 23 with LXDE spin. The desktop did not operate correctly.
So I installed XFCE desktop. (I do not recall how I did so. I had to research how to do it.)
I installed a few apps (firefox, libre office, evince, gimp, scribus, pdfmerge, pdfshuffler), and did nothing major.

I thought "repo" is short for "repository". As I have only dealt with Fedora, I do not see how that is relevant.
Please tell me if it is, as I consider myself a newbie.

do you get an error on a update ?

I am looking at the system now, and trying to understand the impact of using dnf.
Wikipedia says dnf became the default upgrade to yum starting with Fedora 22, yet my install (Fedora 23) is using "Yum Extender".

What the heck; I will have a go:
After doing "dnf clean all", the result was "20 files removed".
After doing "dnf update", the result was "Dependencies resolved. Nothing to do. Complete!"

I then ran Yum Extender again, and "Network Manager" still came up as out-of-date, with the same error.

Maybe this is because of something mentioned on fedoraforum.org =
Here's the bug report: https://bugzilla.redhat.com/show_bug.cgi?id=1342335

I am attempting to understand it now.

did you try rebuilding the rpm database