title: Replace values in column by factor level
body: <p>I got a survey data.frame they are 100 columns and each columns have 2 factors - Yes or No. However some survey have answers like, Yes! or Nope or Yay or Nah... which really they are yes or no.</p>
  
  <p>My question is how can I achieve my converting all values in other columns based on their factor level? e.g if factor level is 1 replace text to Yes else No.</p>
  
  <p>My second question is, sometimes I am left with the 3rd level that isn't used, how can I remove all unused factors in ALL columns in data frame. I got more than 100 columns.</p>