I think op needs to clarify the problem.
If op means that they don't know how to set a static address on sys-net,
then your advice is right. The answer is to open the NetworkManager
applet, select the relevant NIC and the change ip_v4 settings to use
"static" method and enter the relevant IP details.
But I suspect that *that* isn't op's problem.
It reads to me as if they want to change the Qubes internal IP, i.e the
10.137 addresses.
It *is* possible to do that. I do it myself.
First, take a backup of /usr/lib/python3.5/site-packages/qubes/vm/mix
Second, take a backup of /usr/lib/python3.5/site-packages/qubes/vm/mix
Stop all qubes which use a netvm.
Delete the contents of mix/__pycache__
Edit net.py and change every instance of 10.137 to (e.g) 192.168
Restart.
Done.
This is, of course, a hack and unsupported. You may get into all sorts
of trouble. If you do, restore your backup, reboot, and all should be
well.
An alternative approach would be to do some clever work with nftables
and routing to make sure that sys-net routes sys-firewall one way and
all other IPs outbound. Provided there's no conflict between the
sys-firewall IP and "real" IPs on the network that should work.
I like using my own network ranges better. (One less Qubes fingerprint)
unman