The file is /etc/shadow, there will be lines like:
user:asdfasdfasdf$$$$asdfasdfasdfsadfasdf::17178:0:98899:7:::
anotheruser:asdfasdfasdf$$$$asdfasdfasdfsadfasdf::17178:0:98899:7:::
systemd-timesync:*:17199:0:94499:7:::
systemd-bus-proxy:*:17199:0:94449:7:::
messagebus:*:17199:0:93459:7:::
ntp:*:17199:0:23499:7:::
(...)
You must remove the second field as:
user::17178:0:98899:7:::
Then boot, press enter on prompt for password and once login in, change password with passwd.
