Randy, as Jim said, the larger nibs for the Gents, Statesmen, Empero, Imperial & Lotus just screw out the old nib, feed & housing as an assembly & screw in the new assembly in the front section.
For the smaller one, just pull out the nib from the feed housing and insert the new nib. You just need to make sure the feed is in the middle of the nib and that the nib & feed are seated all the way back. You can practice taking your steel ones apart and put them back together, not hard at all.
Here is an old link to the parts
http://www.penturners.org/forum/topic.asp?ARCHIVE=true&TOPIC_ID=6707&SearchTerms=nib,disassembly