try update first and if rowcount = 0 then you need to insert.

SET NOCOUNT OFF;
set XACT_ABORT ON;
-- LEAVE NOCOUNT OFF IF SET TO ON THEN THE ROWCOUNT CHECK WILL FAIL.

 update [dbo].[mem_Favorites]
  set isActive = @isActive
  where mem_id = @mem_id and mem_favorite_id = @mem_favorite_id
  
-- if rowcount is  0 then it must be an insert

if @@ROWCOUNT = 0
 begin
  INSERT INTO [dbo].[mem_Favorites]
      ([mem_id]
      ,[mem_favorite_id]
      ,[DateAdded]
      ,[isActive])
   VALUES
      (@mem_id
      ,@mem_favorite_id
      ,GETDATE()
      ,@isActive)
 end