Decrypt (How to get there: In SecurDat in the 'Options' menu click on 'Decrypt ...' and then select files, or drag and drop files or a folder on the SecurDat main window while holding down the Ctrl key.

If the Overwrite files option is activated, then the encrypted file will be overwritten with the decrypted file. But be careful: If something goes wrong, the encrypted file cannot be restored (unless you have a backup copy of the backup file ;-)
Alternatively you may set a folder in the Output directory where the decrypted files will be saved. You can select this folder with the button, or you can set a subfolder (see example image), where the decrypted files will be saved.

After having input the correct password in the Enter password field you can click on the OK button (mask it with the Mask password option), which starts the decryption process. When finished SecurDat displays a message where you can open a report. (For example, if you have used a wrong password this will be mentioned in the report, etc.).