This can sometimes be difficult, as you’ve seen. To manage this, I recommend right clicking the existing section titles and choosing Remove Section. (you might get a way with doing this only to the first section.) This should remove the section but not remove the contents of the section. Insert your new information, then select each portion of the page in turn and assign new sections, titles, etc.
If you work with sections for a while, you will probably get better at this and more careful to make sure there is a line between sections so new sections can easily be inserted in the future without using this workaround.
I would also investigate using a tabbed table instead of sections. Tabbed tables are pretty handy for providing web page navigation. Adding and removing rows are easier to manage using the table structure.