Print current forwarding servers:

show dns forwarding statistics

Remove current and add new DNS forwarders:

configure 
delete service dns forwarding name-server <\curent\>
set service dns forwarding 2606:4700:4700::1111
set service dns forwarding 2606:4700:4700::1001
set service dns forwarding 1.1.1.1
set service dns forwarding 1.0.0.1
commit
save