in /etc/network/interfaces, where it says
iface wlan0 inet dhcp
replace it with
iface wlan0 inet static
and add these lines immediately below
address YOUR_ADDRESS_GOES_HERE
netmask 255.255.255.0
replacing YOUR_ADDRESS_GOES_HERE with the desired IP.
By the way, it can often be easier and more reliable to do this static assignment of IPs on the router side, by assigning fixed IPs to each board. This way the router's DHCP will also avoid assigning those statically allocated IPs to other devices that may connect to the network, which may be an issue with the Bela-side solution suggested above.
