Update some (here photos) app's registry from outside the app itself.

PrivateUwpRegistry.cpp

#include <windows.h>
#include <userenv.h>
#include <atlbase.h>
#include <stdio.h>
#include <shlobj.h>

#include <string>

#pragma comment(lib, "userenv")

int main()
{
	PWSTR localAppDataPath;
	auto hr = SHGetKnownFolderPath(FOLDERID_LocalAppData, 0, nullptr, &localAppDataPath);
	if (SUCCEEDED(hr))
	{
		std::wstring settingsPath = localAppDataPath;
		CoTaskMemFree(localAppDataPath);
		settingsPath += L"\\Packages\\Microsoft.Windows.Photos_8wekyb3d8bbwe\\Settings\\settings.dat";
		wprintf(L"path:%s\n", settingsPath.c_str());

		HKEY key;
		auto err = RegLoadAppKey(settingsPath.c_str(), &key, KEY_ALL_ACCESS, 0, 0);
		if (err == ERROR_SUCCESS)
		{
			DWORD index = 0;
			do
			{
				wchar_t name[256];
				if (RegEnumKey(key, index, name, 256) != ERROR_SUCCESS)
					break;

				wprintf(L"key:%s\n", name);
				index++;
			} while (true);

			HKEY state;
			if (RegOpenKey(key, L"LocalState", &state) == ERROR_SUCCESS)
			{
				DWORD index = 0;
				do
				{
					wchar_t name[256];
					DWORD size = 256;
					DWORD type = 0;
					if (RegEnumValue(state, index, name, &size, nullptr, &type, nullptr, nullptr) != ERROR_SUCCESS)
						break;

					wprintf(L"value:%s type:0x%08X\n", name, type);
					index++;
				} while (true);

				const DWORD REG_APPX_SETTINGS_BOOL = 0x5F5E10C; // this is the special registry type for boolean
				wchar_t boolText[] = L"true";
				DWORD size = sizeof(boolText) + sizeof(FILETIME); // AppX settings.dat values are values + timestamp
				auto bytes = new BYTE[size];
				CopyMemory(bytes, boolText, sizeof(boolText)); // includes terminating zero
				GetSystemTimeAsFileTime((LPFILETIME)(bytes + sizeof(boolText)));
				err = RegSetValueEx(state, L"HeicCodecMissingTeachingTipStatus", 0, REG_APPX_SETTINGS_BOOL, bytes, size);

				RegCloseKey(state);
			}

			RegCloseKey(key);
		}
	}
	return 0;
}