A very experimental hackety hack to export ePubs from LaTeX on Overleaf, using tex4ebook. After creating a new project from this template and when compilaton has completed, download the generated .epub file using the steps described here.
In this sample set-up, the .epub export will only be triggered if the project is set to compile with pdfLaTeX and will likely fail with many packages; this is a very basic example and this is a hackety-hack.
See the tex4ebook Github repository for documentation and discussion; though bear in mind that . Overleaf does not necessary have the most up-to-date tex4ebook release.