The assignment letters can be found under the section 'Accountant info' in the relationship file. If the assignment letter is made in AdminPulse, click on 'More actions' behind the relevant document. Select the option 'Replace assignment letter' and browse to the new document. When the upload is complete, click on 'More actions' again and click 'Mark as signed'.
Otherwise, you can upload an assignment letter as .pdf that was already signed and which wasn't made in AdminPulse. These will automatically be marked as signed.
More information about assignment letters and assignment letter templates can be found here.