Modifying a [CRC32] checksum protected file
One fundamental property of CRCs is that if you exclusive-or in the bit representation of the polynomial (which, for standard CRC-32, would be 0x0104C11DB7) anywhere in the message, you don't modify the CRC.

So, the obvious thing for you to try is to exclusive-or that value (and similar, for example, the bit-reversed version of above) and see if that modified version has the same CRC.

Once you have verified the CRC polynomial (and the order in which bits are processed), then it is fairly straight-forward to perform arbitrary changes.