Ubuntu – How to change the shortcuts in mplayer

Assuming you are talking of mplayer itself, according to the man page¹:

MPlayer has a fully configurable, command-driven control layer which allows you to control MPlayer using keyboard, mouse, joystick or remote control (with LIRC). See the -input option for ways to customize it.

...

 -input <commands>
          This option can be used to configure certain parts of the input system.  Paths are relative to ~/.mplayer/. Available commands are:  
...
             conf=<filename>
                  Specify input configuration file other than the default ~/.mplayer/input.conf.  ~/.mplayer/<filename> is assumed if no full path is given.
             keylist
                  Prints all keys that can be bound to commands.
             cmdlist
                  Prints all commands that can be bound to keys.

Edit ~/.mplayer/input.conf and add the following line (using the examples from the mplayer website):

RIGHT pt_step 1

¹ I am not linking to the Ubuntu manpage because it differs from what I actually have on my system. In particular, for me, mplayer options began with a single hyphen and not double hyphens.