Any suggestions how to help a soprano who 'hoots' on higher notes? Not extreme high notes, just the normal upper range.
"Hooting" can mean a whole lot. I'd guess you mean a strong resonance not matching the vowels to be produced. Voice production consists of the voice box in the throat and the vocal tract. A "hoot" likely means an excessive resonance chamber being formed in the vocal tract.
Now this is actually pretty tricky since at least for classic singing volume and healthy production depends on resonance since it allows letting the vocal folds work against a comparatively large acoustic resistance, thus moving comparatively little and transferring a comparatively large amount of energy.
So the principal mechanism you associate with hooting is actually desirable but one needs to work the resonance into a better sounding way by not leaving room large enough for "hot potatoes".
One "rule of thumb": tongue in front pushing against the teeth. You don't want parts of it to bunch up in the middle part (or even the back) of the mouth cavity except where needed for vowel formation. And vowel formation is pitch dependent since your resonance (which is felt more in the "mask" or even projected in front of you) should not fall apart when you change vowels. So vowel production, particularly for a soprano, is actually quite unnatural and pitch dependent if you want to produce consistent tonal quality.
Vowel modification is not as conspicable as you would think if it is done consistently since hearing will adapt like sight adapts to changing lighting.
At any rate, this is nothing you can fix as an amateur with some Internet advice: this requires continued feedback of a good and experienced vocal coach. It's not a basic problem with a simple fix but concerns the overall balance and blend of vocal production and technique over the registers of an individual singer.