We have to select the Account field "Select Account Field" drop-down and Account Update Field from the "Select Intermediate Account Update Field" drop-down on STEP-3 on the About Update Client Details Page and then click on the Map button.

P.S. Discover Update Client Details, the easy way to keep client information up to date in Salesforce. Find it on AppExchange - https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EcsATUAZ