Just like in PBasic for the Stamp, you have to do the work yourself one character at a time. Think of strings as byte arrays with just the slightest bit of syntactic sugar (in the form of string/strlength/strcomp). A number of people have been thinking about a string manipulation library, but it's not been done yet. There was a string library "object" posted here some time ago, but it also does some dynamic memory allocation which makes it a little more complicated in some ways than a simple C-like string library.
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http://forums.parallax.com/showthread.php?p=587930
http://forums.parallax.com/showthread.php?p=611308