The most effective, but also the slowest method to upscaling an image without loosing a considerable amount of quality without connecting to the internet is through Adobe Photoshop.
You will need to learn a lot of concepts including, but not limited to bicubic interpolation upscaling, antialiasing, using different brushes and so on. It can take anywhere between 5 minutes to several hours of your time depending on how detailed your image otherwise is and your skill level. For example, it can take no more than a few minutes of your time to edit and upscale an anime or cartoon-related image. However, if you take an image of something like a real person’s portrait, it can take a few hours.
You can also use Adobe Photoshop’s special Preserve Details feature as well as resample to enhance image size. However, since most of it is basic artificial intelligence, you won’t be seeing very good results, as they are more consistent than anything else we might be discussing later onwards.