Thanks for the question. On Canva, there's no actual way to increase your DPI when you resize your image, however, to get the best print quality, when you are done with your design. Click share in the top right corner, there you can select specific details like RGB or CMYK, and there you can choose the best printing options. For the best print quality art save the work with pdf print rather the the other options given. And set the color mode to CMYK. For the second question yu asked, it's best to sell your print on demand wall art in Pdf.

They will be color changes when you Convert CMYK to RGB. RGB has more colors to it than CMYK does, so you might notice darker or dull colors on your converted designs

It's available only for photoshop