Yarinaoshi Koujo no Madou Kakumei: Shokei Sareta Akuyaku Reijou wa Horobiru Kadon wo Tatae Naosu

Falsely accused of attempting to assassinate her fiancé, the Crown Prince, Countess Leticia and her entire family were executed. She was then reborn in Japan, retaining the memories of her past ambition to become a mage-engineer, and achieved success as an engineer — but then she died in a traffic accident. And now… she's been sent back to her 12-year-old self, before the engagement!? There's no way she'll repeat the same mistakes. An engagement? Absolutely not! She'll use her skills to develop new magical artifacts to uncover the conspiracy and save her family!

(Source: Hifumishobo, translated)