locate the Bought transaction for when the shares were originally according to your Quicken records. Edit that transaction accordingly.
It is possible that the shares got into Quicken by an Add Shares transaction, in which case that is the transaction to edit.
Looking at the Security Details reveal that acquiring transaction. Double clicking that transaction shown there should open the transaction up for your edit.
It is a good idea to have a current backup at hand when changing historical transactions, just in case.
