title: replace certain number in DataFrame
body: <p>I am pretty new to Python programming and have a question about replacing certain conditional number in a DataFrame. 
  for example, I have a dateframe with 5 days of data in each column, day1, day2, day3, day4 and day5. For each day, I have 5 data points with some of them larger than 5 for each day. Now I want to set the data which is larger than 5 to 1. 
  So how can I do that? Loop into each column and find specific element then change it, or there is other faster way to do it?
  Thanks,</p>