I don't recommend you do this. The Navigation Pane is super useful. You should assign CtrlH to NavPane so you still have access to it. but here's how you do it.


To remove ctrlF from NavPaneSearch:
Options – Customize ribbon – keyboard shortcuts customize (button) – Categories pick home Tab – Commands pick NavPaneSearch – current keys click on Ctrl+F – hit remove button

To assign Ctrl+f to findReplace:
Options – Customize ribbon – keyboard shortcuts customize (button) – Categories pick home Tab – Commands pick EditReplace – put cursor in “press new shortcut key" and press Ctrl button and F button then hit the assign button