You're looking for a DirectX 12 offline installer for Windows 10 64-bit. Here's what I found:
You can download the DirectX 12 redistributable package from the Microsoft Visual C++ Redistributable for Visual Studio 2015, 2017 and 2019 page. Select the "DirectX 12" option and choose the 64-bit version. directx 12 windows 10 64 bit offline installer link
DirectX 12 is a built-in component of Windows 10, and it's not available as a separate offline installer. However, if you're looking to update or install DirectX 12 on your Windows 10 system, you can try the following options: You're looking for a DirectX 12 offline installer
If you're looking to install DirectX 12 on a system that doesn't have it (e.g., a older version of Windows), you can download the DirectX 12 redistributable package from Microsoft's website. However, please note that this package is meant for developers and not for end-users. DirectX 12 is a built-in component of Windows
Unfortunately, there isn't a straightforward offline installer for DirectX 12. However, you can use the DirectX Redistributable tool from Microsoft, which can help you create an offline installer.