help me to update ios 13.1 in my iphone in order to download some apps

If you're referring to an iPhone 6s, iPhone SE, or any newer model, use a computer or the Software Update function in Settings to install 15.7.2 or 16.2. Otherwise, it's not capable of 13 or above; try downloading the applications to a newer one on the same Apple ID first.

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To update to the most recent, compatible version of iOS, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information.