1. Request certificate. I saved my certificate as unifi2020.crt and unifi2020.key
2. Replace certificate on unifi controller

openssl pkcs12 -export -inkey unifi2020.key -in unifi2020.crt -out unifi.p12 -name unifi  -password pass:temppass
keytool -importkeystore -deststorepass aircontrolenterprise -destkeypass aircontrolenterprise -destkeystore /var/lib/unifi/keystore -srckeystore unifi.p12 -srcstoretype PKCS12 -srcstorepass temppass -alias unifi -noprompt

3. Restart unifi

/etc/init.d/unifi restart