Minecraft wiki entry for Updating_old_terrain, associated text: Download and install MCEdit. The recommended version is MCEdit 2.0.0 beta 12. The download link is for Windows and Mac.
Download the biome deleter plugin here and extract it. You will find two files; a copy of this tutorial, as well as the plugin in a file called 'biome_deleter.py'.
Install the plugin by copying it into the plugin folder.
On Windows, you will find a folder next to the MCEdit 2 exe called 'MCEdit 2 Files'. The 'plugins' folder is inside that folder.
On macOS, the plugins folder is '~/Documents/MCEdit 2 Files/plugins'. You might need to create that folder if it doesn't exist.
Download and install MCA selector.