i need to handle text in a shape with thousand of characters

when i insert a character or a string by character location,
the insert method replace the string from position 10 to 265 with "a"
it is not like an insert method
ActiveSheet.Shapes("Text Box 17").Characters(10).insert("a")

how to do the insert task correctly??



and seems i can not save more than 255 characters in a string
but as in documentation, a string can store up to billions of characters

dim temp as string
temp = ActiveSheet.Shapes("Text Box 17").Characters(Start:=6,
Length:=256).text

is this part of the code have error instead of the string?
ActiveSheet.Shapes("Text Box 17").Characters(Start:=6, Length:=256)

What i can think about is to cut the string part by part.. and merge it
finally.
Is there any better way to do this?