How to edit AAL template region

Hi all,

I have a quick question and I was hoping somebody might be able to help me. I have an a priori hypothesis about the involvement of Wernicke's area in a group of patients, and would like to use the AAL template regions to create an ROI. The AAL regions for middle temporal gyrus and superior temporal gyrus both span the entire length of the gyrus from anterior to posterior, but I was wondering if there is a way to isolate or edit the region to only look at the posterior third of the gyri. I have been looking online but haven't found any specific advice. 

Thanks in advance for the help!