Change character in string using Mid$?

So, I know that Mid$ returns a character(s) based on it's numeric range. But how would I go about changing one particular character within a string without disturbing the others?

Example: There are 10 items in a stage. Each of those items has a flag represented as a 1 in the string. When collected, the 1 needs to become a 0 so when the stage reloads after a death, the item does not spawn.

Found a way to do this.

Quote

+Upon pressing space bar
-Start loop "SaveItems" NObjects( "Items" )

+On loop "SaveItems"
+ID ("Items") = LoopIndex("SaveItems")
-Change Alterable Text to string$(Global)+Str$(Collected("Items")