If you had the TOC generated by the TOC tool, you can change the level there using the arrows

Use the Saved Search (I assume current NEW installs still have) to demote H1 or H2 to H3 (assumes Chapters, the most common, are will be H3), then use the TOC tool to promote the Part Title to H2
(I only use H1 for (Omnibus) Book Title Level)
You may need to adjust your syylesheet if it called out H1, H2 or H3 for the various titles