iOS 13

What model of iPhone do you have? See the following from iOS 13 - Apple

iOS 13 is compatible with these devices.

• iPhone 11
• iPhone 11 Pro
• iPhone 11 Pro Max
• iPhone XS
• iPhone XS Max
• iPhone XR
• iPhone X
• iPhone 8
• iPhone 8 Plus
• iPhone 7
• iPhone 7 Plus
• iPhone 6s
• iPhone 6s Plus
• iPhone SE
• iPod touch (7th generation)**

Why can't you update to iOS 13? What version of iPhone do you have? You have to be connected to wi-fi, then go to Settings>General>Software Update. See this for trouble with over the air updates, Get help with over-the-air iOS updates - Apple Support