Empirically I learned that the VBA function Mid() can be used on the
left side of an assignment. For example:

s = "12345678"
Mid(s, 4, 2) = "cd"

results in "123cd678". But the following does not work:

Left(s, 2) = "ab"
Right(s,2) = "ef"

Then I looked at the Help text for Mid, and I found nothing that
indicates that even Mid() can be on the left side of an assignment.

Where is written that I can use Mid() on the left side?

Is there something else that I should use to overwrite any part of a
string?

(Other than cumbersome concatenation like Mid(s,1,3) & "cd" & Mid(s,
6,3).)

Is there an easier way to overwrite the left and right parts of a
string other than using Mid()?