I second the Thanos link, but if you want to only use Prometheus, I would look into federation:

https://www.robustperception.io/scaling-and-federating-prome...

https://prometheus.io/docs/prometheus/latest/federation/

You can set up a new "cold storage" Prometheus with a longer retention that scrapes select metrics from your regular 30d Prometheus to store for longer periods of time.