As said in the title. My customer request 300 dpi, how to change it in GimP or photoshop? Thanks
300 dpi means probably your image must have 300 pixels per inch when the image is shown in its intended size. Another way to say the same is "the image must have resolution 300 pixels per inch"
Dpi as a term is from printing. There it tells how big are the smallest dots in a print process, but I guess your client tries only to ensure the images have enough pixels to look sharp in his applications. It has become a common habit to demand 300 pixels per inch because it stands some zooming in without looking too pixelated on the screen and it's enough for many common print processes.
The other common habit is to call the quantity ppi (=pixels per inch) "dpi". Your client seems to have that habit, because otherwise he should specify the print process and not demand something from you and your images.
If your image will have in its intended final usage width = W inches and height = H inches, you must start the image in the beginning of the job with pixel dimensions width=300xW pixels and height=300xH pixels. If your image happens to be just 300 pixels high and 300 pixels wide, it cannot be shown in bigger size than 1 inch x 1 inch or you have less than 300 pixels per inch.
For convenience programs have the new image starting dialog built with a possibility to set the wanted ppi and the intended final display size in inches or millimeters. The intended display size will be stored in many image file formats such as JPG so that image viewing software can show the image just in the right size.
Often from cameras one gets more pixels than he needs for 300 ppi.
Images can be resampled to other pixel dimensions. That happens when you activate in Photoshop Image > Resize and allow resampling. Without allowing resampling only the intended display size changes. I guess your client gets angry if you try to increase pixels per inch of an already existing image by making the pixel dimensions bigger. That's because resampling images to bigger pixel dimensions do not make them sharper. It's no more than only zooming in except the file will be bigger.
Reducing pixel dimensions to make file size smaller is common. It removes details, but images which are watched only on the screen and will be never zoomed in can well have only 72 ppi.
But if you have a 72 ppi screenshot or you have saved a 72 ppi image from a web page, do not expect you will get anything usable if you go to image resize, set new ppi=300 and allow resampling. Your image has formally the numbers right, but the information hasn't increased. The result will be as blurry and pixelated as you would get by zooming in the original 72 ppi image.
The only way to get usefully 300 ppi is to have it or more when the image was born. If you happen to have say 600 ppi, for example your image is intended to be shown as 5 in x 5 in and the pixel dimensions are 3000 x 3000, you can reduce the pixel dimensions with resize to 1500 x 1500 without breaking the 300 ppi rule. But then it starts to look blurry when it's watched as 10 in x 10 in. Originally it could have been sharp even in size 10 inches x 10 inches.
In GIMP you must open subdialog "Advanced" in the new image starting dialog to get a possibility to set the wanted ppi.
In GIMP you change the pixel dimensions with Image > Scale dialog. It's a little different than Photoshop's Image > Resize. Resampling happens if you change the pixel dimensions, there's no need to check option "Resample"
The problem how to increase pixel dimensions so that the apparent sharpness will not suffer has many solution attempts. Special image enlargening software such as Smilla Enlarger (=free) or On1 Resize (=commercial) try to guess where is a sharp border in the image and do not make it wider. I have in many cases made a printable copy from a low resolution image with them, but as often they guess wrong or they cannot bring up that detail which is nonexistent but still essential.