حَيْثُ can express a place and may also be used for time. Its final dammah remains fixed.
فَكُلَا مِنْ حَيْثُ شِئْتُمَا وَلَا تَقْرَبَا هَذِهِ الشَّجَرَةَ
So eat from wherever you wish, but do not approach this tree.
- Here حَيْثُ answers from where?, so it expresses place.
- The following clause شِئْتُمَا completes its meaning.
- Do not change the ending of حَيْثُ.
Recap: direction words, prepositional place phrases, direct bounded-place nouns, قَبْلُ / بَعْدُ, and حَيْثُ all require the full sentence for correct interpretation.