How to edit a string via slice

You can't access an index of a string directly, but I did discover something:

stringy:String="something"
Print stringy
Print stringy[0..4]
stringy = Stringy[0..4] + "test" + Stringy[4..9] 'you can concatenate slices and strings interchangably apparently
Print stringy

*EDIT*

Although the EndIndex appears to change and become a length parameter instead, I don't know if this is intended functionality or not.