Moving Back Is Allowed
In Klondike Solitaire Turn One, you are absolutely allowed to move the top card of any foundation pile back to an appropriate tableau pile. This can be a game-saving maneuver when you've accidentally "buried" a card you now need to build a sequence.
Strategic Tradeoff
While it's allowed, moving a card back from the foundation is usually a sign that you're in a tough spot. It can reduce your current score and potentially slow down your progress toward a win. However, if it unlocks a hidden card in the tableau, it's almost always worth the sacrifice.
Try Undo First
Before manually moving a card back, try using the Undo button. It's often cleaner to simply reverse the move that sent the card to the foundation in the first place, avoiding any scoring penalties associated with moving cards "downward."