HELP! How do I change the dpi of this image in photoshop to 300?

[discreetthinker]

I created a t.shirt design which I needed to print, but I got back a response from the t.shirt person and they said that they need the image to be 300dpi in order to print. How do I do that? It's just resolution right? Will I have to start over? Thank you!!!!

 

[iDad]

what is it at now? It depends on the picture, what type of image?

 

[yaro]

I would start from learning the difference between dpi and ppi
yaro