I have iPhone SE using 12.3.1 How should I update iOS? It has an error updating to 13.5.1

See this Apple support article—>If your iPhone, iPad or iPod touch won’t update - Apple Support

What is the specific problem you have when you go to Settings/General/Software Update and tap Update Now?

Here are the conditions that must be met to update iOS:

• You must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• The phone can not be in Low Power mode
• The battery state of charge must be above 50%
• You must have at least 3 GB of free storage
• You cannot be connected through VPN
• If you are not planning for it to update automatically you should turn this option off, as it will only update when the phone is plugged in, connected to Wi-Fi and locked
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).