mjhjmtu

This is my routing table when I connect my Android phone via USB to my Raspberry and enable USB-tethering.

$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.42.129 0.0.0.0 UG 204 0 0 usb0
0.0.0.0 192.168.0.1 0.0.0.0 UG 303 0 0 wlan0
192.168.0.0 0.0.0.0 255.255.255.0 U 303 0 0 wlan0
192.168.42.0 0.0.0.0 255.255.255.0 U 204 0 0 usb0

I want wlan0 to be the preferred interface.
So how can I change the metric of one the interfaces permanently?

Also how does Linux decide which metric value it will use for an interface?

One of the solutions is to configure settings in /etc/network/interfaces adding default route with a predefined metric:

auto lo
iface lo inet loopback

# Primary interface
auto wlan0
iface wlan0 inet static 
 address 192.168.0.100
 netmask 255.255.255.0 
 dns-nameservers 192.168.0.1 8.8.8.8
 post-up /sbin/ip route add default via 192.168.0.1 dev wlan0 metric 10
 post-down /sbin/ip route del default via 192.168.0.1 dev wlan0 metric 10
 wpa-ssid <your_SSID>
 wpa-psk <your_PSK>

Maybe, you need to add another Wi-Fi options specific to your settings.

Another solution is to change the metric in the /etc/dhcpcd.conf.
According to the dhcpcd manual metric can be assigned to the interface:

metric
 Metrics are used to prefer an interface over another one, lowest
 wins. dhcpcd will supply a default metric of 200 +
 if_nametoindex(3). An extra 100 will be added for wireless
 interfaces. 

Add these lines to /etc/dhcpcd.conf:

interface wlan0
metric 200

interface usb0
metric 300

And restart dhcpcd and networking services.