I couldn’t update my phone, what should I do?

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

These are the iOS options based on iPhone Model --> Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

• iOS 13.5.1 —> 6S / 6S Plus, SE, 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max
• iOS 12.4.7 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, iPad Air, iPad Mini 2, iPad Mini 3 and iPod Touch (6th generation)
• iOS 10.3.4 —> Final release supported on iPhone 5 and iPad (4th generation) Wi-Fi + Cellular
• iOS 10.3.3 —> Final release supported on iPhone 5C and iPad (4th generation) Wi-Fi
• iOS 9.3.6 —> Final release supported on iPhone 4S, iPad 2 Wi-Fi + Cellular, iPad (3rd generation) Wi-Fi + Cellular, and iPad Mini (1st generation) Wi-Fi + Cellular
• iOS 9.3.5 —> Final release supported on iPod Touch (5th generation), iPad 2 Wi-Fi, iPad (3rd generation) Wi-Fi, and iPad Mini (1st generation) Wi-Fi
• iOS 7.1.2 —> Final release supported on iPhone 4.
• iOS 6.1.5 —> Final release supported on iPhone 3GS, and iPod Touch (4th generation)