MID$ Statement - DualBrain/bsharp Wiki
To replace a portion of one string with another string.
Both n and m are integer expressions.
stringexp1 and stringexp2 are string expressions.
The characters in stringexp1, beginning at position n, are replaced by the characters in stringexp2.
The optional m refers to the number of characters from stringexp2 that are used in the replacement. If m is omitted, all of stringexp2 is used.
Whether m is omitted or included, the replacement of characters never goes beyond the original length of stringexp1.
10 A$="KANSAS CITY, MO" 20 MID$(A$, 14)="KS" 30 PRINT A$
KANSAS CITY, KS
Line 20 overwrites "MO" in the A$ string with "KS".