Sure it is very easy to change from command line. Something like:
asa>en
asa#config terminal
asa(config)# interface eth0/1 (or your interface currently asigned the address)
asa(config-if)# ip address
asa(config)# end
asa# write memory
This can also be done from the ASDM GUI. See "Configuration, Device Setup, Interfaces". Choose the outside interface and "Edit".
You need also check for things like static NAT addresses whihch may also use addresses from the public IP block. It is easiest from the command line, thus:
asa#show run | i
That will show you all the lines in the config that inlude addresses from your external network number.
Hope this helps.