How to change programs used as default?

Question: I have installed another web browser. How do I change it so that it is the one used as default?

Answer: Edit /etc/defaultprograms with your favorite text editor and change the entries there. To change default browser from "seamonkey-spot" to firefox (without spot), for example, change the DEF_BROWSER entry in that file and replace the text within quotes. You can do the same for the rest of the applications listed there: draw, html editor, paint, media player, etc.

Since Fatdog64-710 Beta you can also use “Fatdog64 Edit Default Programs” GUI available in Control Panel on “Desktop” tab. This method changes the default applications on per-user basis (~/.fatdog/defaultprograms).

Note: Doing this will only change the defaults for programs that honour Puppy-like model of default programs.
Unfortunately there are no single unified way to change the "default" programs unless you run a comprehensive desktop environment like KDE or Gnome. Every program has its own way of keeping track of what it thinks as the default.

XDG-compliant programs, for example SpaceFM, make use of /usr/share/applications/mimeapps.list and /usr/share/applications/defaults.list (see for details). Other file managers like XFE or ROX-Filer has its own way of keeping default programs. You will have consult the documentation of each individual program if the method described above doesn't seem to work.