Good morning Vanessa,
you have 2 options:
post a “Buyer Request” (on the main fiverr page, from your top menu bar, choose Buying > Post a Request)
go through the category Writing & Translation > Proofreading & Editing yourself and look for a seller that seems to meet your requirements, has good reviews, a sensible price etc. (hint: if the price sounds far too good to be true, it might be; compare the sellers/gigs you think are okay, you can “favorite” gigs, so you´ll be able to find them back quickly)
Some sellers specify in their gig descriptions that they will only work with specific files, usually MS Word, or you need to contact them before ordering to discuss details, which I would recommend to do either way because working directly in WordPress is a specific requirement.
I don´t do proofreading, but translation, and I do translate directly in WordPress for some customers, who create accounts and passwords for their different translators to log in, so it is doable and certainly you can find proofreaders who are okay with it.
It is absolutely recommendable to make a backup of your WP site before you let anyone else log in!
Hope that helped a bit. Good luck with your project.