First of all, DPI is just a simple piece of information inside the image. It's an instruction to the printer on how many pixels of the image should be used per inch, both horizontally and vertically. Unless you decided to make an image 300 x 300 pixels, you probably have more than enough pixels in your image for a decent size print at 300 DPI. Let's have a look:
At 8" x 10", which is a pretty common photo print size, you would need 2400 x 3000 pixels in your image to print full-sized at 300 DPI. It's the math: 8" x 300 DPI = 2400 pixels, so too is 10" x 300 DPI = 3000 pixels. For comparison, the photo dimensions from an iPhone 6s or newer is 3024 x 4032. Yep, native print size from an iPhone 6s is about 10" x 13.5" (give or take a fraction or so). So, yeah, you probably have all of the information in your image for a reasonable print.
So, what do you do in the case of misconfigured DPI? Well, most photo editing apps, including Photoshop, GIMP, Affinity Photo, and a host of others, allow you to change the DPI of an image. The trick, and this is critical, is that you do not want to resample the image, this would increase the pixel dimensions and, quite likely, look super terrible. So make sure you're looking for that option to disable when changing DPI.
For futures, I highly recommend creating a bunch of standard presets in your application of choice. Apps like Procreate allow you to do this readily, and having them already predefined helps to prevent a mistake.