It is a config word setting. Here is an example:
2.1 Oscillator Types

PIC18F2525/2620/4525/4620 devices can be operated
in ten different oscillator modes. The user can program
the Configuration bits, FOSC3:FOSC0, in Configuration
Register 1H to select one of these ten modes:
1. LP Low-Power Crystal
2. XT Crystal/Resonator
3. HS High-Speed Crystal/Resonator
4. HSPLL High-Speed Crystal/Resonator
with PLL Enabled
5. RC External Resistor/Capacitor with
FOSC/4 Output on RA6
6. RCIO External Resistor/Capacitor with I/O
on RA6
7. INTIO1 Internal Oscillator with FOSC/4 Output
on RA6 and I/O on RA7
8. INTIO2 Internal Oscillator with I/O on RA6
and RA7
9. EC External Clock with FOSC/4 Output
10. ECIO External Clock with I/O on RA6