I have an exist /23 subnet range that I want to split into 2 x /24 subnet ranges. Is there any way to edit the exist /23 range or is the only option is to delete and recreate both subnets ranges?
There's no "GUI way" to do this. While you CAN edit the object manually (ConsoleOne, iMan, LDAP) it'll likely be easier and more error-proof to delete / recreate. Remember that it's not just about a subnet object but likely also about pools.
Are you using DDNS?
This is another reason to delete / recreate subnets and pools. I'd suggest to reduce the lease time to let's say 15 or 30 minutes a few days before you start splitting. If you schedule a downtime (with all clients down) the DDNS entries can cleanup on their own and you won't have to fight with leftovers.