You can renew your Basic / Platform / Insights Platform subscription through the self‑serve “extend subscription” flow:
When your account is close to expiring, you’ll see an option to extend your subscription in the account/billing area of the Planet Insights / Platform interface.
Follow that link (or navigate to the same “plans & packages” section from within your account), choose the same Basic plan (or another available plan if you want to change), and complete the renewal.
If you don’t see the renewal option, or if you need a formal quote/invoice or want to adjust your plan, you should contact Planet Support or Sales so they can handle the renewal or provide a budget/quote for you.