Giant decorative grasses clobbered by rain
We recommend taking no action now, partly because the flopping is not harming its ability to photosynthesize (and it will be going dormant relatively soon, in another month or two) and partly because staking may be more trouble than it's worth and not result in as tidy a look as you may be envisioning. (Ornamental grass foliage like this usually has sharp serrations that easily cut skin when a person is trying to wrangle the foliage into a support.)
You can cut it back to within a few inches of the ground, but wait until late winter or early spring to do this. If this is a mature clump, which it appears to be, the center may be dead - this is a natural growth pattern for these grasses as they mature, but can only be remedied by division. Around the time you cut it back, dig up and divide the clump as best you can - it can be an arduous task with large established grasses - and discard the dead center portion. Any site that is very sunny, well-drained, and doesn't have overly-rich soil (not high in organic matter or nutrients) will suit them well and promote growth that is resistant to flopping in the future.