Website to manually download iOS

There is no means to do that, other than to update your iPhone or iPad. That you’re not updating implies that you’re using an older device; one lacking support for current iOS or iPadOS. iPhone 6 or earlier, for instance. There’s no means to update an unsupported device to a newer and unsupported version.

If you are on a device with support for current versions, here is how to update:

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

You might want to borrow and use a Mac ir Windows computer for the update, if you’re on a very old iOS version. How to do that is included in the linked update article.