hi! Depends on the program you are using, but if you use image editing softwares (Photoshop, Krita, Gimp etc) you have to find scale image-option, and use 'Nearest Neighbour' filter, so it doesn't get blurry. Usually pixel art scales x4, so the pixels don't get distorted. Here's a tutorial on photoshop.
But you can scale the image up in game editing software also, Here's a tutorial for Unity. You can find good tutorials on youtube by searching for exaple terms 'Upscaling pixel art in (the software you are going to use)'.
Hope this helps, let me know if that doesn't work or if there's a problem!