Language files for all Joomla components, plugins, etc are stored in the same place. In the location of your top-level Joomla folders (components, plugins, images, etc) you will see a folder "language". Inside this folder will be a folder representing your language, or multiple languages that you have setup for your site. Inside that next folder are the actual files. So for DT Register, it is (for English):
/language/en-GB/en-GB.com_dtregister.ini
You can modify these language files a couple different ways:
1) Use FTP. With a program like WS_FTP, CuteFTP, FileZilla, etc... you can login to your hosting account to gain access to the files. Your web host would provide the login info needed for this. Then you'd see your local computer files stacked up next to the server files for your website. Pull the language file to your local computer so you can edit it using any text editor like Notepad, WordPad, etc. Personally, I like to use PSPad. After making the change, transfer it back to the hosting account using the FTP program.
2) Use a file management component like extPlorer or NinjaXplorer. They are free Joomla components that work like a FTP program, just all within Joomla. That may be easier for you if FTP is a foreign thing to you. Same deal though... go to the file, edit, save.
So what exactly do you change??
In the language file, you will see a long list of language tags that look something like this:
DT_GROUP_REGISTRATION=Group Registration
This means that the code creating what you see on the page does not have hard-coded the text you actually see. It just has "DT_GROUP_REGISTRATION". Then if you change the language file, every place in the component that uses that tag will change... instead of having to change multiple places. So instead of "Group Registration", what if you want to say "Team Registration"? Just change that one line in the language file...
DT_GROUP_REGISTRATION=Team Registration
Edit whatever lines you want/need to, save, reload the modified file to your site and you're set! If you need to change anything on the backend administrator side, there is another set of language files:
/administrator/language/en-GB/
Hope that helps!