It might not be possible. If you're using the TimeGridView from SGDEX that uses the TimeGrid component, and I don't see a documented way to switch that to 24hr format.
https://developer.roku.com/en-ca/docs/references/scenegraph/list-and-grid-nodes/timegrid.md
Sometimes you can delve around inside the built in components at runtime to unsafely modify things, but usually not in lists & grids.
