Use this case statement in a snippet with a rating parameter to change numerical ratings to stars:

case
  when ROUND([rating]) = 5
    then '★★★★★'
  when ROUND([rating]) = 4
    then '★★★★☆'
  when ROUND([rating]) = 3
    then '★★★☆☆'
  when ROUND([rating]) = 2
    then '★★☆☆☆'
  when ROUND([rating]) = 1
    then '★☆☆☆☆'
  else null
end