OK you need to get the position of the second "\" and third "\" characters.
The position of the first character is given by
x = Instr("#description#x:\directory\subdirectory\filemane", "\directory\"
so the position of the second "\" is just
y = x + 10
Now if we use the first position
after the second "\" as a starting point, we can find the value of the third "\"
z=Instr(
y+1,"#description#x:\directory\subdirectory\filemane", "\"
So if we put that into code, this is what we have:
In the OnChange event of the Subdirectory field on the form, put the following
dim strHyperlink as string
dim x as int
dim y as int
strHyperlink = me!HyperlinkFieldName
'get position of second "\"
x = Instr(strhyperlink, "\directory\"

+ 10
'get position of third "\"
y = Instr(x+1,strhyperlink, "\"
me!HyperlinkFieldName = left(strHyperlink,x) & me!subdirectory & mid(strhyperlink,y)
This does not have any error checking and it is off the top of my head, so you will probably have to tinker with it. Check out the help files on InStr, Left and Mid.
Hope this helps.
Kathryn