Since the release of PrestaShop v1.0 or later, you have been able to move modules up and down (or possibly left or right) within a section of the page. For example, modules within the leftColumn can be moved up or down to change the order by using positions. You can also move certain modules to different parts of a page, from the left column to the right column for example and/or limit their appearance to certain pages.

To install a new PrestaShop module:
/modules folder.Once configuration is complete, be sure to test the module immediately to confirm that it functions as planned. If it does not, also check the positions Tab (which describes where the module is installed).
Tip: When installing new modules (or if you create your own modules) remember that all modules have to be converted to the .tar format before they can be added to Prestashop via ftp. To do this, you need a program that can generate .tar zipped files such as “7 zip” which is free. If you use 7zip, when you right click on your folder, to compress 7zip will ask you which format you want to use eg .zip, .rar. Choose .tar and your .tar file will then be generated which you can now upload to your shop upload using an FTP program. Install the module in your modules directory.
Do remember only to zip the folder that contains your files, so not zip any sub-folders. Foe example, if you module is called mymodule and inside this is another folder called mymodule2 with your files in it, zip only this mymodule2 folder, not the outer mymodule folder. And make sure, when you uplaod any files the correct permissions are set (if you have existing modules you are using in your store, these will have the corect permissions).