str = "cat"
 str[0] = 'h'
puts str

---> now using ruby 2.2.2-p95 tagged as `222p95`
PS C:Userslogin> irb
irb(main):001:0> str = "cat"
=> "cat"
irb(main):002:0> str[0]
=> "c"
irb(main):003:0> str[0] = 'h'
=> "h"
irb(main):004:0> str
=> "hat"
irb(main):005:0>