Quoted from http://maven.apache.org/settings.html:
There are two locations where a settings.xml file may live:
The Maven install: $M2_HOME/conf/settings.xml
A user's install: ${user.home}/.m2/settings.xml
So, usually for a specific user you edit
/home/*username*/.m2/settings.xml
To set environment for all local users, you might think about changing the first path.