Lowering PIC Clock Frequency
If you're currently running at 48 MHz, you're using an external crystal. You can decrease the frequency by changing bits 2:0 in the PLLDIV register, but we'd need to know the external crystal frequency and what peripherals you want to adjust. Is it just the USB subsystem you need to use?

In response to your comment:

Looks like you can get it down to 5 MHz with no hardware changes. Take a look at Table 2-3 the datasheet. Switch clock mode to disable the PLL and set CPUDIV1:0 to 11 and you'll get a 4x divider down to 5 MHZ.