If that is the exact query you're running, then I imagine it's this line:

when [Tim].[dbo].[PP_FFR_BU].[Zip] = [TimCIS].[dbo].[Legis_Districts].[Zip5] and [Tim].[dbo].[PP_FFR_BU].[Zip4] <> [TimCIS].[dbo].[Legis_Districts].[Zip4]

The TimCIS.dbo.Legis_Districts table is not in a FROM clause for the UPDATE, so it can't be referenced except in a subquery that does specify that table in a FROM clause.

Cheers!