Just like any other extension. At Ext. 10, Feature 00, Left Intercom Twice, Right Intercom Once, dial the extension number that you want to change, dial **, press the lines in the order that you want them to be accessed, dial ** again, Feature 00.
You will find many pointers in the FAQ section of the Partner section of
http://support.avaya.com , and you can also download the entire manual.