C++, best way to change a string at a particular index

C++, best way to change a string at a particular index

I want to change a C++ string at a particular index like this:
string s = "abc";
s[1] = 'a';
Is the following code valid? Is this an acceptable way to do this?
I didn't find any reference which says it is valid:
http://www.cplusplus.com/reference/string/string/
Which says that through "overloaded [] operator in string" we can perform
the write operation.