From your Letsencrypt ACME host copy fullchain.cert and unifi key file
scp fullchain.cert unifi.mife.ca.key unifi.mife.ca:/root/
From your Unifi host create a new keystore
In my case the unifi is the only key in the keystore, so you can just generate a new keystore with new certificates ** This assumes the unifi keystore is located in /var/lib/unifi/keystore (ubuntu/debian)
cp /var/lib/unifi/keystore /root/keystore.bak
openssl pkcs12 -export -in fullchain.cer -inkey unifi.mife.ca.key -out /var/lib/unifi/keystore -passout pass:aircontrolenterprise -name unifi
Restart Unifi service
systemctl restart unifi