I'm not sure that you can.  The connector for the speaker to the motherboard is not your typical connector for speakers but something more like the usual computer cables inside the machines.  You may be able to replace the speaker itself, though, with something more powerful.

I think the internal speaker is a very low impedance so you'd need some kind of pre-amp connected. Probably could be done with an old 741 OP-Amp circuit to drive a slightly larger speaker for more output.

It occurs to me that maybe some cheap PC speakers with the two inputs soldered together and the common and those two wires connected to the speaker plug which goes to the motherboard would probably work as it takes input from a line out jack which is also not supposed to drive a low impedance speaker directly.