(Python) I want to replace a string'array element ，but have a problem :  I want to replace a string'array element ，but have a problem:

this is correct!!
[enter image description here][1]


however，this is error:
 
[enter image description here][2]

it seem replace first char....
thanks to your help
  [1]: https://i.stack.imgur.com/KgAQY.png
  [2]: https://i.stack.imgur.com/cIKGb.png