It’s not letting me update to it

Update to what?

If your iPhone 12 Pro Max is not letting you update to iOS 18, that's because iOS 18 is not out yet. If you are having difficulty updating to the latest version of iOS 17, see

Updating iOS: common issues and solutions - Apple Community

If your iPhone 12 is still on iOS 16, you have skipped a number of updates, so the best way to update to the current iOS 17.6.1 is to use your computer. Follow the directions in this link --> Update software on iPhone, iPad, or iPod touch in iTunes on PC - Apple Support