Depending on the platforms, migration tools might exist but if this is not the case or the time to install and configure the software to accomplish this outweighs the amount of data, you might think about migrating manually. Mmmm, not my kind of preferred job so here the steps I take to get the job done a little quicker …
Make a list of all HTML pages and images with Screaming Frog SEO Spider.
Import the HTML pages list in a Google spreadsheet and use the IMPORTXML function with the right XPath expression to import the URL’s main content. To find the latter, you can inspect the content element on the page with the Chrome Developer tools. Titles are already detected by the Screaming Frog SEO Spider. Export as CSV file and import into your new platform.
To bulk download all images if you don’t have any access to the files directly, install the Chrono Download Manager Chrome extension and add the image url list to download all in bulk. Uploading to the new platform, speaks for itself, I guess.