Open the trust and click on Associated Parties.
Select the trustee you will be retiring, place a tick next to the person / corporate trustee company. Click on cogs and select edit
Scroll down to Roles and Associations
Place tick in "Remove as Trustee"
Select the Date of removal. In this example the date is 07 Nov 2014
Scroll down to Access to this trust, if the person / corporate trustee company is no longer having access to the trust, remove all ticks.
When you highlight the person / corporate trustee company the trusteeship now shows the dates the person was a trustee within the trust.