Which free tool improves video quality?

Fix what can actually be fixed first. Microsoft Clipchamp's free editor adjusts brightness, contrast and colour and adds filters, then exports watermark-free up to 1080p; DaVinci Resolve's free version is the professional tool for colour correction. Making a low-resolution video sharper is a different job, upscaling: Video2X is a free, open-source machine-learning upscaler that runs on your own computer, and Topaz is the best-known paid option, with Topaz for Web's video plan listed at $29 a month. Upscaling invents detail, so it helps a soft old clip more than a badly lit one. Sound usually matters more than pixels, so clean the audio too. If your video only looks soft after posting it to YouTube, the problem is often the export: keep the frame rate you recorded in and follow YouTube's recommended upload settings (H.264, variable bitrate). Rule: fix light, colour and sound first, and upscale only when the source really was low resolution.