Short answer: Open Customize home (bottom of Home or Profile) and scroll to NAVIGATION. Under Your bottom bar, drag the slots to reorder them, or tap a slot to swap in something from your Dashboard. Whatever you replace moves to the Dashboard, so nothing is lost.
The bar always holds four items. Everything else sits in the grid that opens from the round Dashboard button. When you swap something into the bar, the item it replaces moves into the Dashboard grid. The Tools button under the grid is always there.
To undo your changes, tap Reset to defaults at the bottom of Customize. It restores the default bottom bar and cover color and leaves your widgets and theme alone.