i downloaded free 30 days trial of adobe but i cant understand how to use it?i want to chnge background?

I assume you mean you downloaded the free trial of Photoshop. In order to do what you want to do, you have to understand about layers. If you don't, I suggest you consider getting a basic book (you might even try your local library), Photoshop for Dummies (no, it is no an insult, the Dummies series of books is for beginners, people who need to know the basics, without being technical).

You should see a Layers palette when you open an image in Photoshop. Befoe you do anything else, do a Save As and give the file a new name. ALWAYS DO THIS so you have working copy and keep the original.

That first layer is probably named Background and locked (you will see a little lock icon next to the thumbnail of the layer in the palette). Double click on the layer in the palette to unlock it. You will get a little dialog box that will say Layer 0 (that's the default name when you unlock the background layer). Click ok. Now open an image you want as your background. Click Ctrl +A to select all of it (Command +A on a Mac). You will see the "marching ants" dotted line selection border around the image. Go to your Tools palette (by default that's on the left side of your screen). Up at the top is the Move tool, it looks like an arrowhead with an odd sort of cross symbol. Click on that and then click and drag the image you've selected across into the other image.

It will come in as the new Layer on top of Layer 0. This is okay. You won't see Layer 0 in your picture because you dragged the new background on top of it. Go to the Layer palette, you will see Layer 1 on top of Layer 0 in the palette. Click on Layer 0. At the bottom of the Layer Palette is an icon that looks like a page with a folded corner. This is the New Layer icon. Drag Layer 0 onto that icon to duplicate it. You will see a new Layer called Layer 0 Copy. Click on that and drag it ABOVE the Layer 1. Now you will see in your image that the original image shows again. You have two copies in your image of that original, indicated by Layer 0 and Layer 0 Copy in the Layers palette.

Now you can use the Eraser tool (it looks like an eraser, in the Tools palette) if you wish (this is the easiest way, though not always the best) to erase the parts of the original image where you want the new background to show through. Note that when you use the Eraser tool, you can change the size by going up to the top and clicking in the bar where you see a brush or tool symbol. You will get different sizes to select, and whether its hard or soft.

This is not complete instructions for this, but it will get you started. Note that it is not the only way to do this. There's almost always more than one way to do something in Photoshop. But first it is a good idea to learn what the tools are and why Layers are so important.

I bet you're using the wrong adobe program.