tsql how to update column in all records of a table ..

/****** Script for SelectTopNRows command from SSMS  ******/
SELECT top 2 p.ProgramID,a.ProgramIDM
  FROM [radiomix].[dbo].[chat] a
 inner JOIN [radiomix].[dbo].[MixFmProgram] p ON (a.DAY = p.ProgramWeekDay)
   WHERE 
  CONVERT(varchar,[datetimein], 114) >= convert(varchar, CONVERT(datetime,p.ProgramStart), 114)
and CONVERT(varchar,[datetimein], 114) < convert(varchar, CONVERT(datetime,p.ProgramEnd), 114 )
 order by a.id desc

Select allOpen in new window

this is the querry
i want to update all p,ProgramIDM to the a.ProgramID


iin this example i have selected only 2 record order by a.id desc
i want to update the value of programidm in table chat by the correct one ??
what should be the query ??