Do I have to go through every update from 12.4 or can I update directly to the latest version?

You can update directly to iOS 12.5.6.

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.6

• If you have an iPhone 6S through 13, you can update to 15.6.1

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone, iPad, or iPod touch - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support

You can update directly to the highest version that your phone is eligible for-Check compatibility:

Go to Settings/General/About and note the Model Name

• NOTE: iOS 16 is not available for any models except to iOS developers and beta participants. General release will be in Fall, 2022
• iPhone 12 or 13 series: You can update to 15.6.1
• Any model 6S or newer (including original SE, 2nd and 3rd editions): You can update to 15.6.1
• iPhone 5S or 6: You cannot update to iOS 13 or higher; you can update only to iOS 12.5.6
• iPhone 5: You can update only to 10.3.4
• iPhone 5C: You can update only to 10.3.3
• iPhone 4S: You can update only to 9.3.6
• iPhone 4: You can update only to 7.1.2