Hi,
Define priority, is it inbound or outbound or both? with BGP you have to treat Incoming/outgoing traffic as a "virtual circuit" each with its own rules.
Assuming both ISPs are different, then you'd need to:
1.- For outgoing traffic, set a LOCAL_PREFERENCE above 100 for all the routes coming from the ISP you want to be used as main, here if you're receiving more than a default route, I'd suggest you to only get it and not full tables, since all the traffic would anyway reach internet using always the same path unless there's a problem.
2.- For incoming traffic you could use many things each having a different success rate, for instance you could use:
A.- prepending, do take into account that this doesn't work 100% of the time, AS can choose what path to use at will even if you explicitly tell them that's not prefered.
B.- Publish a more specific prefix over main ISP and the aggregate over the secondary ISP, this always work (at least I haven't seen any tricks to bypass it), but this requires you to have at least a /23 to publish, so main ISP would get 2x /24 and secondary would get just the aggregated /23
C.- Use some sort of BGP conditional advertisement so you never broadcast the prefix to ISP2 unless some conditions are met (for instance ISP1 detecting as down), check this https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/16137-cond-adv.html as long this is done correctly, should always work since to start with the world wouldn't see you via ISP2
HTH
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