Increasing or upscaling video resolution, converting SD video to HD for instance, refers to scale up the video image to fill a HD format (always with high resolution). It increases more pixels, but the quality will not improve. Meanwhile, it will change video size to larger one. So a poor original video file (e.g. 320P) is ALWAYS going to get bad result after you change video resolution to HD or 4K. Whatever software you use, it MUST NOT reach your expected results. However, you might be satisfied with the final effect if you convert from 320P to 480P, 480P to 720P, 720P to 1080P, 1080P to 4K.
On the contrary, a huge demand also exists to reduce video resolution (aka downscale video resolution), such as downscale 4K to 2K, 1080P, 720P, 480P and so forth. Because the higher resolution means it needs higher configuration to run and more space to store. That's the reason someone always wants to convert 4K to 1080P, 1080P to 720P.
Finally, the tool you use is also an important factor. No matter whether you change video resolution online or offline, you need to make ensure the tool is qualified enough rather than an advertising stunt.