From the Help file
Save changes to a document
On the File menu, click Save. To save the file with a new name, on the File menu, click Save As, type a different name in File name, and then click Save. Documents that use multiple languages should be saved as rich text files.
WordPad automatically saves documents as rich text files, but you can change the default file type at any time. On the File menu, click Save As. In Save as type, select the document format you want to set as the default. For example, to always save documents as text files, click Text Document. Select the Save in this format by default check box, and then click Save. This saves the current document in the selected file format and sets the default file format for future documents until you change it again.