Drivers for hardware devices are normally uninstalled in Add/Remove Programs in Control Panel.
After doing that, remove the old modem from Device Manager, shut down the computer, remove the old modem from the system, install the new one, restart, and install its drivers.
If the new modem does not work properly, then see if it's using the same IRQ that's being used by another hardware device in Device Manager. If so, then manually change the modem's IRQ.
If no IRQ's are available and your system has unused COM(serial) ports, then you can free up IRQ 3 and/or 4 by disabling the serial ports in BIOS, and the modem may be able to use one of these.