According to Apple's notes, the update fixes a bug that caused clips you didn't mean to select to be included when selecting multiple clips using the Shift key, and it addresses issues related to XML import and export. Full notes:
What's NewToday's update comes three weeks after the release of Final Cut Pro 10.4.1, a major update that introduced a new ProRes RAW format, new closed captioning tools, and enhanced tools for exporting files.
- Fixes an issue in which selecting multiple clips using the Shift key or marquee selection could inadvertently select other clips in the timeline
- Fixes issues related to XML import and export
Final Cut Pro can be downloaded from the Mac App Store for $299.99. [Direct Link]