The following command in the terminal will map the left, small button to middle button, making it the "paste"-button.

xmodmap -e "pointer = 1 4 3 6 7 2 5 8 9 10 11"


You can see the result by typing

xmodmap -pp

There are 11 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