is it possible to edit a sentence ive published online?
Yes, correct your InDesign document and re-upload it.
Derek's steering you right. Publish Online is a lot like posting to a website. Actually, pretty much exactly like posting to a website.
It's a snapshot in time. In and of itself, it's not editable. But it can be updated by fixing the InDesign source document, then posting the fixed document to Publish Online to create a new snapshot in time with your corrected information.
What if my InDesign source document has disappeared. The only thing that exists now is the pdf online.
If the original InDesign document you used to create the Publish Online document does not exist anymore, sadly, you'll have to recreate the InDesign document again.
Also, to add to Derek's answer, when you re-upload it, but sure and check "Update Existing Document" IF you want the URL to stay the same.
And if you only have a PDF, I suppose you could edit the PDF and drop the pages into InDesign, then publish it.
Of course bear in mind, the PDF is a static version (as Diane will know better than me, being a world-class expert on the subject of Publish Online).
Yes, absolutely as Derek says, PDF pages would not be editable in InDesign.
Just depends on how complex it would be, or necessary it would be, to re-create the ID file.
There is also a third party product, PDF2ID that converts PDF to InDesign: https://www.recosoft.com/products/pdf2id/