Adding to the end.

I have a recording and I simply want to "record over the last 5 seconds with something different. How does this work? I seem to always take myself back to the beginning

Audacity always records on a new track unless you do “append-record”

If you click in the first track where you want to record new stuff then click “Record”, Audacity will record on a new track from that point.
If you make a selection in the first track then click “Record”, Audacity will record on a new track for exactly the time and location you have selected.
To record over the end of a track, first delete the portion you want to record over, then click “Record” while holding the Shift key, or press Shift+R.

Watch out for overdub and latency settings which may cause the newly-recorded section to be off by a fraction of a second. Of course, you can always use the editing and time-shift tools to put the newly-recorded section exactly where you want it.

There are several sections in the manual that will help you get started with recording and overdubbing.

– Bill