Version 6.5 modifies the database schema. Do not simply reinstall v6.4 over it without restoring the old DB.
The v6.x series focuses on modernization and performance. Key highlights include: Enhanced Performance ultimate-pos-v6.5.zip
Why upgrade to the version found inside ultimate-pos-v6.5.zip ? The changelog usually highlights: Version 6
: Includes a dedicated POS screen for quick billing, support for credit sales, quotations, and suspended sales [4, 10]. support for credit sales
Resolved issues such as negative values being entered in quantity fields and category switching glitches on the POS screen. Installation and Security Warning UltimatePOS - Ultimate Fosters