How do I replace a GFCI breaker with a normal breaker?
First, it sounds like the old breaker is a GFCI or AFCI, and the new one is not. Changing types like that is likely a code violation and can compromise the safety of your electrical system. Do not do this unless you know why a GFCI or AFCI breaker was installed and that you have removed the conditions from that branch circuit that required their installation.

Second, since you are asking this type of question, you likely don't have the experience to do this safely. At this point, I recommend you hire an electrician.

If you insist on continuing and have confirmed that it is safe to remove the GFCI or AFCI breaker, then start by tracing the white wires. One should be going to a bus bar in the breaker box and the other should be exiting the box with the black wire. Disconnect the one wire from the bus bar, and connect the other one to the bus bar in its place.