Worked out how to do this. See Below.
PRINT
BEGINNING:
PRINT "What number VLAN is you Machine in?"
PRINT
INPUT vlan$
PRINT "You entered VLAN "; vlan$; " - is this correct? y/n"
x$ = INPUT$(1)
IF x$ = "n" OR x$ = "N" THEN GOTO BEGINNING:
LET vlan1$ = "1 1 " + vlan$ + " 1"
'OPEN "c:\SCRIPTS\PROTOCOL.UPD" FOR INPUT AS #2
OPEN "i", 2, "c:\SCRIPTS\PROTOCOL.UPD"
OPEN "o", 3, "c:\SCRIPTS\PROTOCOL.CHG"
WHILE NOT EOF(2)
LINE INPUT #2, a$
IF a$ <> "DefaultGateway0=changeme" THEN PRINT #3, a$
IF a$ = "DefaultGateway0=changeme" THEN PRINT #3, "DefaultGateway0="; vlan1$
WEND
CLOSE #1
CLOSE #2
OPEN "i", 4, "c:\SCRIPTS\PROTOCOL.CHG"
OPEN "o", 5, "c:\SCRIPTS\PROTOCOL.INI"
WHILE NOT EOF(4)
LINE INPUT #4, a$
IF b$ <> "DefaultGateway0=changeme" THEN PRINT #4, b$
IF b$ = "DefaultGateway0=changeme" THEN PRINT #4, "DefaultGateway0="; vlan1$
WEND
CLOSE #3
CLOSE #4