Styles Administration: phpBB templates
In order to change the template of your forum, you will first need to download the desired template's files to your local computer. They usually come as a compressed file, so make sure you unpack that on your machine. The result should be a folder with the template's name, containing two other folders and a number of files.
When the extraction is done, connect to your web hosting account using FTP. The FTP client will display you the files and folders you have on your website, and there you will be able to upload the new theme. Open the www folder and then the directory, where your forum is located. Look for the templates folder and upload the extracted content of your archive there.
Wait until all the files are transmitted to your account.
When the upload is complete, please go to the Styles Admin menu. Select the Add link. Your newly uploaded template will appear. Click the Install link.
Next, please go to the Configuration link in the General Admin menu. Scroll to the Default Style option and select the new template you have just installed.
When done, please go to the bottom of the configuration screen and click the Submit button in order to make your new template active. Enjoy!