One of the buttons on my laptop broke so to avoid that key I switched to using a left hand mouse. Here are the codes for each key:

xmodmap -pp  
There are 12 pointer buttons defined.

    Physical        Button
     Button          Code
        1              1
        2              2
        3              3
        4              4
        5              5
        6              6
        7              7
        8              8
        9              9
       10             10
       11             11
       12             12

And here I switch the left and right buttons:

xmodmap -e "pointer = 3 2 1 4 5 6 7 8 9 11 12 13"