I found some information in the website, that:
• Unrestricted Software License
• The PIX 515E Unrestricted (PIX 515E-UR) model extends the capabilities of the family with support for stateful failover, additional LAN interfaces, and increased VPN throughput via integrated hardware-based VPN acceleration. It includes an integrated VAC or VAC+ hardware VPN accelerator, 128 MB of RAM, two 10/100 Fast Ethernet interfaces, and support for up to four additional 10/100 Fast Ethernet interfaces. The Cisco PIX 515E-UR also adds the ability to share state information with a secondary Cisco PIX Security Appliance (either in an Active/Active or Active/Standby deployment model) for resilient network protection."
So, how can I change to Unrestriced Sofeware License mode so that i can use 4 more Fast Ether interface???