If you are already in France and your stay extends beyond your current visa/residence permit validity, you must handle either (A) VLS-TS validation or (B) renewal of your residence permit, depending on your situation.
A) You have a VLS-TS (Long-stay visa acting as residence permit): validate it after arrival
If your visa says VLS-TS, you must validate it online within 3 months after arriving in France.
Validation is done on the official government platform (ANEF):
Official procedure (Service-Public): https://www.service-public.fr/particuliers/vosdroits/R52684
Official reminder (France-Visas): https://france-visas.gouv.fr/en/your-arrival-in-france
What you typically need:
Passport + VLS-TS details
Date of entry in France
French address
Online payment method for the tax (if required)
B) Your document expires soon: renew your residence permit (titre de séjour)
If your stay continues beyond the expiration date, apply well in advance (do not wait for the last weeks).
The renewal process is usually done online via ANEF: