iPhone 12 update crashes during installation

There is part of your problem. Apple recommends the battery be replaced when the capacity reaches 80%. Lower than that can have effects on device performance. You can try charging the device up to 100% and keeping it plugged in when you try to update. Have you tried by over the air and by connecting to a computer?

"iPhone 12 update crashes during installation: My iPhone 12 won’t update I don’t know why I downloaded the new version of iOS than I clickdd on install now than apple logo appears on my screen than disappears instantly and I must download the update again a again pls help me guys I don’t know the problem"

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Fixing a Boot Loop:

Your iPhone is apparently stuck in a Boot Loop. This typically occurs when you go abuut installing an update, and it stalled while in the process. Use my User Tip: How to Fix an iPhone in a Boot Loop - User Tip

"I don’t have a computer but thanks for advices this is my first time with this problem"

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You Need a Computer:

Ask a friend or colleague if you may use their Mac or PC for this. Use iTunes for older macOS versions and Windows. Finder will perform this in newer macOS versions.

It seems that post was deleted. I went to update the last post I did and it wouldn't accept and found that the thread was gone.

For the OP, you next step would be Apple Support. Official Apple Support

Also, check Settings>Battery, and report the Battery health and Capacity.

Your best bet at this point would be to contact Apple Support or an Authorized Apple Service provider to have your battery replaced. Genius Bar Reservation and Apple Support Options - Apple

Here is a User Tip you could review and see if anything there helps. Updating iOS: common issues and solutions - Apple Community

At the same time they could assist you with updating your iOS.

Don't post duplicates. Continue with your original post. No one wants to go back and forth to determine what other information you have already been provided and what steps you have followed.

iPhone 12 update incomplete and crashes - Apple Community

Yes, I see that now also and another reason why duplicate posting is a bad idea. I tried to address it, but now it is too late.