Hello Mehnaz

There are many ways to download the ASTER DEM data

Here I will give you details how you can download it from Earthexplorer website. https://earthexplorer.usgs.gov/

First you have to register for free in the website.

After registration you can login to the main website and can download DEM.

Here is the procedure how you can download the product.

1. you have to select the area of interest from the search criteria using the the map giving in the site.

2. you have to select DATE (of interest) of the product in the date range criteria.

3. click on data sets

4. in the second page you will find different data products in which digital elevation is one of them...please click on it.

5. from different elevation products ASTER DEM is at first option. please check it (tick it in the box) and go into the results at the bottom line.

6. from the results you will get the DEM of interests....in the bottom of each image there is a download option.

DOWNLOAD and here your are done.

HOPE IT WILL BE HELPFULL

Thanks