How do I restart airflow webserver?

Can you check $AIRFLOW_HOME/airflow-webserver.pid for the process id of your webserver daemon?

Then pass it a kill signal to kill it

cat $AIRFLOW_HOME/airflow-webserver.pid | xargs kill -9

Then clear the pid file

cat /dev/null >  $AIRFLOW_HOME/airflow-webserver.pid

Then just run

airflow webserver -p 8080 -D True

to restart the daemon.

This worked for me (multiple times! :D )

find the process id: (assuming 8080 is the port)

lsof -i tcp:8080

kill it

kill <pid>

I advice running airflow in a robust way, with auto-recovery with systemd
so you can do:
- to start systemctl start airflow
- to stop systemctl stop airflow
- to restart systemctl restart airflow
For this you'll need a systemd 'unit' file. As a (working) example you can use the following:
put it in /lib/systemd/system/airflow.service

[Unit]
Description=Airflow webserver daemon
After=network.target postgresql.service mysql.service redis.service rabbitmq-server.service
Wants=postgresql.service mysql.service redis.service rabbitmq-server.service
[Service]
PIDFile=/run/airflow/webserver.pid
EnvironmentFile=/home/airflow/airflow.env
User=airflow
Group=airflow
Type=simple
ExecStart=/bin/bash -c 'export AIRFLOW_HOME=/home/airflow ; airflow webserver --pid /run/airflow/webserver.pid'
ExecReload=/bin/kill -s HUP $MAINPID
ExecStop=/bin/kill -s TERM $MAINPID
Restart=on-failure
RestartSec=42s
PrivateTmp=true
[Install]
WantedBy=multi-user.target

P.S: change AIRFLOW_HOME to where your airflow folder with the config