assuming nothing changed in its API, you should be able to download p5.min.js from that page and place it on your board at this location: /root/Bela/IDE/public/js/ . To do that through the IDE, simply drag-and-drop p5.min.js into the current project and run the following in the console at the bottom of the IDE:
mv /root/Bela/projects/PROJECT_NAME/p5.min.js /root/Bela/IDE/public/js/
after replacing PROJECT_NAME with the name of the project that you dropped it into.