For disambiguation purposes, see also Insert (Config).
In BR, in order to insert one string into another, we must use the desired starting subscript and ZERO as the ending subscript:
00010 dim a$*255 00020 let a$ = "beginning end" 00030 let a$(10:0) = " middle " ! here we are inserting 6 characters (" middle ") starting at character 10 of string a$ 00040 print a$
Note that there is a difference between inserting characters and replacing characters in a string. When replacing, some characters from the original string are removed. When inserting, no characters from the original string are removed.