Hi De Rossi Gianni,
Please refer below administrative guide for installation:
Also check with SAP notes:
1712788 - Installation of RSP 2.4 PL00 - PL05 Fails If RSP 2.1
Database Exists on Target Machine
1819711 - RSP: How to reinstall a damaged RSP
installation
Hope helpful.
Thanks & Regards,
Nagarajan