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The most basic way to alter the contents of an existing string is with
aset (see Array Functions). (aset string
idx char) stores char into string at index
idx. Each character occupies one or more bytes, and if char
needs a different number of bytes from the character already present at
that index, aset signals an error.
A more powerful function is store-substring:
This function alters part of the contents of the string string, by storing obj starting at index idx. The argument obj may be either a character or a (smaller) string.
Since it is impossible to change the length of an existing string, it is an error if obj doesn't fit within string's actual length, or if any new character requires a different number of bytes from the character currently present at that point in string.
To clear out a string that contained a password, use
clear-string: