: Insert the USB into the target PC, restart it, and tap the boot menu key (often F12 , F10 , or ESC ) to select the USB drive as the primary boot device.

The ( BOOTMGR ) is a built-in software utility that controls the startup process of your computer. You cannot "download" it as a standalone app; instead, it is included as a core component of the Windows 10 operating system and is typically restored using the Microsoft Media Creation Tool or recovery environments. How to "Download" and Restore the Boot Manager

Since the boot manager is a system file, you restore it by creating that contains the necessary files to repair your existing setup.