Replacing characters in a string

Hi,

with a character variable you can manipulte the contents via

var+5(02) = 'XY'.

With a string variable this is not allowed. Is there any other way of doing this except copying the string to a char variable, doing the change and copy it back? Another way of doing this concatenating the first part, the new chars and the rest together, but this is an ugly solution too....

Why is it syntactically ok to read parts of a string, but not to write? Is it because a string content is shared between several variables all pointing to the same string content?