The easiest way to accomplish this is to simply add an Addendum to the Lease which retains all existing terms unchanged except for the changes specified in the Addendum. In this case the lease term.
I would however look into the lease rate, typically a month to month lease runs a little higher than a yearly lease. There could be some discount that goes along with the longer term which you can negotiate.
Feel free to call if you have other questions. My info is in my profile.
Aug 18, 2014