You have to extend your work permit.
1) from inside Canada, send your application to CPC, Vegreville (
http://www.cic.gc.ca/english/information/applications/extend-worker.asp). Since your job is LMO exempt you just need the job offer and supporting funds.
2) since you are an US citizen you can apply for the work permit at the port of entry (as you enter Canada). You would need job offer and supporting funds. (
http://www.cic.gc.ca/english/work/apply-who-eligible.asp#enter)