Extend above can have several antonyms, depending on the context. In a physical sense, some antonyms can be "below" or "underneath." For example, if we are talking about a bridge, the bridge spanned over the river may extend above the water, but the pillars underneath the bridge would be located below or underneath.
In a figurative sense, some antonyms can be "retract," "reduce," or "shrink." For example, if we are talking about a company's profits, the profits may have extended above expectations in one quarter, but in the next quarter, they may retract or shrink below expectations. Similarly, if we are talking about a person's confidence, they may extend above the norm in a particular situation, but in another situation, they may reduce their confidence levels.