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. This is provided 'as is' and is not officially supported by Overleaf.
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.