Hi,
Before upgrading, please check the following,
1. Overview note for SAP Business one 9.0 PL 08 (based on your PL)
2. Check software and hardware requirements from "Supported platform guide for 8.8 and higher". This guide can be downloaded from SAP business one customer portal
3. For installation, refer Administrative guide for installation.
4. Refer blog General Overview End of Maintenance for SAP Business One and Related Products
Thanks & Regards,
Nagarajan