The plugin most commonly used for setting alpha to 255 (making semi-transparent pixels fully opaque) is known as Transparency (part of the BoltBait pack) or Alpha Transform
Layer → Rotate/Zoom → check "Cubic" resampling (not exact)Adjustments → Brightness / Contrast on the alpha channel?If you prefer not to install external plugins, you can achieve a similar effect using built-in features: paint net alpha to 255 plugin download
Happy editing 🎨