1. Find the plugin on the spigotmc/github page. Im going to be using this website as a example Bell Tower | SpigotMC - High Performance Minecraft
2. Download the version of the plugin that is supported by your server type and version
3. Remove the old version of the plugin from your minehut server (via file manager or the addons tab if its there)
4. Upload the file you downloaded in the plugins folder in the file manager.
5. Restart your server If this doesnt work, check your addon tab and remove the plugin there, restart, upload the file, and restart again.