Open the trust and click on Associated Parties.
Select the beneficiary you will be retiring, place a tick next to the persons name. Click on cogs and select edit.
Scroll down to Roles and Associations
Place tick in 'Remove as Beneficiary'
Select the Date of removal, In this example the date is 15 Oct 2013
You will be returned to the main relationships page, left click on the person again. The beneficiary history now shows the dates the person was a beneficiary with the trust.