Objects you added yourself (text, images, shapes, drawings in Insert): click the object and press the Del key.
Existing comments and markups of the PDF (notes, highlights and other annotations): switch to Annotate, click the annotation and press the Del key.
Existing page content (text and images of the page itself): Editing existing content is in the works and will be released with one of the upcoming versions once it is finished. Until then you can remove it with Redact: choose White as the color and drag a box over the area. The content under the box is removed permanently when saving. This also works with https://tools.pdf24.org/en/redact-pdf