Descartes wants to "establish" some "firm and lasting" (or "firm and permanent") knowledge. (IP3 42b)

To do this he will "begin again from the first foundations" or "build anew from the foundation" (IP3 42b)

First he will "raze everything," i.e., will engage in "the general destruction of my former opinions" or "the general upheaval of all my former opinions." (IP3 43a)

He need not show these opinions to be false (IP3 43a), because ...

"I should withhold my assent no less carefully from things which are not plainly certain and indubitable than I would from what is patently false ..." or "I ought no less carefully to wishold my assent from matters which are not entirely certain and indubitable than from those which appear to me manifestly to be false ..." (IP3 43a)

I.e., if you can't be sure of it, don't believe it.

If there is a "reason for doubting" (IP3 43a) a belief, then one can't be sure of it.

And if one can't be sure of "the foundations" (IP3 43a), one can't be sure of beliefs that have been "built upon them," the "edifice." (IP3 43a)

So all Descartes needs to do (for this destructive project) is to show that there is reason for doubting the foundational beliefs.