iPhone 6s - Ios 13.5.1 - Software update was downloaded but cannot install

Hi everyone,

I am using 6s.

The current version is 10.2.1.

I want to update the current software to 13.5.1.

I downloaded the software update. However, every time I tap to "Install update", while or after verifying it, a message appears saying "Unable to Install Update. An error occurred installing iOS 13.5.1".

I have 21,13 GB available. I deleted and redownloaded the update.

I am connected to Wifi and try to install the update while charging the phone and also with full battery (not while charging).

I do not know what else I can do for fixing this problem.

Could anyone give me some advise for updating my phone to iOS 13.5.1?

If my phone is not compatible with iOS 13.5.1, then how can I download the latest software which is compatible with iPhone 6s?

Thank you in advance!