Go to the official website of the developer of the program is or just use any Internet search engine by entering the query "Download updates for Photoshop". In the query or in the search on the website specify your version of graphic editor, for example, "Update Adobe Photoshop CS4".
Select the appropriate update and download it to your computer. Run the file and follow the enclosed instructions. Restart the computer after installing updates. If the app is confusing you, use it trial (free) version. Upon expiration of the free use submit your payment according to the instructions attached to the file.
If after installing all the updates to an existing version of Photoshop doesn't meet your requirements, you will have only one option – download and install a new, more advanced version. To do this, repeat the already described operation, just adjust the query in the search engine "Download Adobe Photoshop", putting the version you need (e.g. Adobe Photoshop CS5).