We define what existence is. We have all collectively agreed that being here in the perceptible world means that something exists. We could choose to include the imaginary in its definition and then would be able to say that dragons and wizards exist. We could also choose to say something has to be present in 4 dimensions to exist, in which case we'd not be able to say that anything exists.
A social construct is simply an idea that has been created and accepted by the people in a society which includes the dictionary itself. I'd also say that these definitions are often useful at allowing us to communicate and cooperate with one another, but that doesn't mean we didn't make up the idea.
If the thing in question is changed, and the only part of us that can directly affect is our opinion, then I would call that a social construction.