When you want to check out a source file and modify it in Dexterity, you must first manually check it out of the repository. If the source file already exists in the directory for the project, it will be overwritten. In Dexterity, you can perform a check out operation, which will read the source file from the project directory and import it into the current dictionary.
Keep in mind that even though Dexterity shows a source file as being Checked Out, no locking occurs in the project directory. Other developers could perform a check out operation and make modifications to the same resource.