irb(main):027:0> s = "Blah"
=> "Blah"
irb(main):028:0> lower = ('a'..'z').to_a.join('')
=> "abcdefghijklmnopqrstuvwxyz"
irb(main):029:0> upper = ('A'..'Z').to_a.join('')
=> "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
irb(main):030:0> s.tr(lower+upper, upper+lower)
=> "bLAH"



s = "Blah"
=> "Blah"
(0...s.size).map do |i|
  case s[i]
    when ?a..?z then s[i]-32
    when ?A..?Z then s[i]+32
    else s[i]
  end
end.map{|x| x.chr }.join('')
=> "bLAH"