you would have to setup routes with different matrics.
route to ISP1 metric of 1
route to ISP2 metric of 250
probably something like
ip route 0.0.0.0 0.0.0.0 10.20.30.1
ip route 0.0.0.0 0.0.0.0 10.30.20.1 250
assuming that 10.20.30.0 was to ISP1 and want to use as the primary and 10.30.20.0 was to ISP2 and uses as the secondary.
I am sure that there are other ways to do it depending on which protocols are being used with the ISPs. like BGP or RIP2 over frame-relay…etc.