The section of Book III in which God claims that he did not fore-ordain the fall of Adam and Eve (even though he had fore-knowledge that they would sin) practically deconstructs itself.

Deconstruction is a form of literary theory or analysis that can be applied to any text or any part of a text. It was immensely popular in English departments in the 1980s and 1990s precisely because it enabled people to churn out essays quickly and mechanically on any possible topic; this is also the reason it has lost traction in the past decade, namely that scholars began it realize that rather than revealing anything about the texts to which it was applied, deconstruction simply led you to a pre-ordained conclusion.

For Milton and literary theory, you might want to look at two books by Stanley Fish, Surprised by Sin amd How Milton Works. They are more grounded in reader response (in Fish's own neo-pragmatist variation) than pure deconstruction, but they have both been very influential.

To do a standard deconstructive move on Paradise Lost,  find pairs of binary oppositions in the poem such as heaven-hell, God-Satan, good-evil. Next, you "interrogate" the "privileged" position. This means you take the position or subject most people believe is better (God, good, heaven) and show (1) that it is actually worse (e.g. Satan more admirable than God), (2) that the two parts of a binary pair depend on each other (you can't have good without evil) and (3) that the distinctions between elements of the pairs are not as sharp as they look (Satan was once an angel, good and evil can overlap, etc.)