The only thing that matters with an image is the number of pixels it contains; “resolution” only becomes a meaningful concept when the image is output to some display medium, such as screen or paper, where it tells the output device how many pixels there are to some display device measurement. You can change the resolution of an image without removing or adding pixels using graphics software such as Photoshop (very pricey). Or you can download some freeware like Gimp, which can do it for free. A bit complex to use, though, if you aren’t familiar with graphics software. You could probably Google on how to do it. Or maybe there is a website that can do it for you…let me check.
Ok, this site:
It allows you to specify a “DPI”, which means Dots per Inch, which is the same as resolution. I increased the resolution of a .jpg file I had to 300 DPI, which should be good enough to print.