Install
Open the server in the panel → tab Plugins (next to Files). Search by name. Results come from Modrinth and SpigotMC. Click Install on the version the installer suggests (it matches your Minecraft version), or open Versions to pick another. The jar lands in plugins/. Restart when you are done installing.
Dependencies (e.g. Vault for economy plugins) are listed after the install with their own Install button.
Update
Search the plugin again; the card shows Installed with the version. Installing a newer version replaces the old jar and keeps the plugin's config folder. Read the plugin's changelog first if you are several versions behind, because configs occasionally change format.
Remove
Plugins tab → Installed list on the right → trash icon. This deletes the jar. Its config folder in plugins/ stays so a reinstall keeps your settings; delete it by hand if you want it gone.
Premium plugins
SpigotMC premium resources cannot be downloaded by the installer. You buy them on SpigotMC with your account and upload the jar to plugins/ yourself. The installer marks them.
Plugin does not load
Console says why: wrong Minecraft version ("Unsupported API version"), missing dependency ("Could not load ... depends on Vault"), or Java version. Ask Kowalski to read the log if the message is unclear.