How can I update may ipad mini 2 to Latest software?Because I couldn't install some apps like Microsoft Office 'cause I only have iOS version 12.5.5

You have 2 options.

1. If you have access to a newer device, you can "purchase" the apps with your Apple ID on that device then download the latest compatible version to your device.
2. If you do not have access to a newer device, you will need to purchase a newer device to use these apps. You device is 7 years old and has served a long and useful life.

Look for other applications that do the same things; in some cases, you can also get a compatible version by downloading the current one to a newer unit on the same Apple ID first.

The second generation iPad mini supports 12.5.5; it's the first that's limited to 9.

(206211)

The iPad mini 2 support ends with iOS 9. I am guessing you have the iPad mini 3, but either way, you can’t upgrade to iOS 15. To upgrade to iOS 13 or higher, you need an iPad mini 4 or newer.

I also have a iPad mini 2 with iOS version 12.5.5. So I’m not sure where you got the information about only being able to get support up to iOS 9.

Correct. The iPad mini 2 can go to 12.5.5. This doesn't resolve the issue, however, as the OP needs iOS 13 or later.