replace data in one file with the data in another file

Question

i'm trying to replace specific data in one file with the data in another file using c

following is my barcode label...

c0001

f260

L

D11

H30

R0000

C0040

1X1100000100010B300300003003

181100202900027Part No

181100202900097[PRTNUM]

1e5504002400030B

1X1100002300010L300003

191100202000030Quantity

191100202000080[QUANTY]

1e5504001500040B

1X1100001400010L300003

1X1100001400150L003090

191100202000170P.O.No

191100202000220[PONUMB]

1e5504001500180B

191100201200030Supplier

1e3304000700030B

1X1100000600010L300003

181100200300030Serial

181100200300090[SERIAL]

171100300900190Rev

171100300300190[REV]

171100300900240Units

171100300300240[UNITS]

1X1100000100180L003130

Q0001

E

& the content in another file is as below

<pre lang="vb">132424235

004342

L1000

DZ12

234235

234235</pre>

i want to replace field [PRTNUM] from first file with data 004342 in another file in this way i want all data inside [] is replaced by data from other file.