If the back glass of your iPhone 12 mini is cracked or damaged and you want to replace it, you have a few options:

1. Apple Authorized Service Provider (AASP): The safest and recommended option is to take your iPhone 12 mini to an Apple Authorized Service Provider. They have trained technicians who can replace the back glass with genuine Apple parts and ensure that your device is properly repaired. However, keep in mind that this might be the most expensive option.

2. Apple Store: You can also take your iPhone 12 mini directly to an official Apple Store for back glass replacement. They offer similar services to the AASP and use genuine Apple parts.

3. Third-Party Repair Shops: There are many third-party repair shops that offer iPhone back glass replacement services. Some of them may use non-genuine parts, so make sure to do your research and read reviews to find a reputable and trustworthy repair shop. Keep in mind that using non-genuine parts may void your warranty with Apple.

4. DIY Repair: If you are experienced with electronics and phone repairs, you can purchase a replacement back glass online and attempt to replace it yourself. However, this is a challenging task, and it can be easy to damage the phone further if you're not careful. Additionally, attempting a DIY repair may void your warranty with Apple.

Before proceeding with any repair option, it's important to check the warranty status of your iPhone 12 mini. If the device is still under warranty, getting the repair done by Apple or an authorized service provider is strongly recommended to preserve the warranty coverage.

Lastly, I want to emphasize that any form of repair or modification to your iPhone should be done with caution. If you are not confident in your ability to perform the repair, it's best to seek professional assistance to avoid causing further damage to your device.