You can open a page in the Page Editor and fill it in with data by directly typing or selecting content.
The content will depend on the type of the page field. For each type there are different editors. See all the supported editors in Page Fields Editors.
To edit page fields
Preview the page in Page Editor.
Click a page field to access that field type editor.
Edit the field as necessary and move on to the next field.
When you are finished editing, click Save.