How can I add or delete route about multi path one by one?

Question

for example, current default route is

ip route add default nexthop via 10.0.2.1 weight 1 nexthop via 10.0.2.3 weight 1

if the nexthop 10.0.2.1 is unreacheable, how can I only del 10.0.2.1, not del all then add 10.0.2.3 only.

if I want add new nexthop 10.0.2.4, how can I do it gracefully not del current route then add new route with new nexthop.