traffic count and 2 wan switching

hi

i have to wan interface with to ISPs. I have prepaid traffic at ISPs.
wan1—ISP1—prepaid 5Gb incoming traffic
wan2—ISP2—prepaid 3Gb incoming traffic

this to interface work in failover mode - wan1 is main and wan2 is backup
i need somehow to switch wan2 as main and wan1 as backup then incoming traffic on wan1 will come higher then 5Gb

have any one some desicion for this? or maybe someone give me an example which help me?

any ideas?