I have used an SSL certificate from Lets Encrypt and my website is now HTTPS, however the certificate expiry is set to 90 days. How can I increase this? Certificates from Godaddy and other vendors seem to be valid for 1 year or longer.
Lets Encrypt has set a 90 day certificate lifetime intentionally. They feel a certificate with a shorter lifetime is more secure, as it limits the damage from key compromise and mis-issuance. And if stolen, the keys and mis-issued certificates are valid for a shorter period of time.
The certificates issued by Lets Encrypt are set to automatically renew. They do not expect system administrators to manually handle renewals. For this reason, the timeline of the certificate should not matter, as it will renew after 90 days on its own.
You can read more about this here - https://letsencrypt.org/2015/11/09/why-90-days.html