Hi,
The automatic backups, which are part of the Azure PostgreSQL services cannot be altered as far as I know.
This means that the 35 days is the max retention available as of today.
If you want a longer retention time, I suggest you post feedback about this over at the Azure Database for PostgreSQL uservoice page over here:
https://feedback.azure.com/forums/597976-azure-database-for-postgresql
Best regards,
Leon