A package can be uninstalled via the Puppy Package Manager, (this doubles up as a way to see what is installed):

• In Lucid Puppy 5.2x - A list of installed packages is shown in the main window, un-install packages by selecting one from this list.
• In Slacko Puppy 5.3x - Click on the 'uninstall' button. A list of installed packages is shown, un-install packages by selecting one from this list.

Slacko screenshot

PETs may not uninstall cleanly. If unsure make a backup of your Pup Save File beforehand (frugal).

SFS packages are unloaded via the BootManager or SFSLoad. As SFS files are read-only and less likely to uninstall uncleanly.

Puppy Package Management
PETs - software packages
SFS - sfs format packages

Guide Installing and Making Pets page
Guide to making PET files on forum
PET does not contain all the files in the folder - thread