When an encrypted archive is open in the main PowerArchiver window, go to Actions and select Remove Archive Encryption.
You will then be presented with the Remove Archive Encryption window that will offer two choices for encryption removal:
Same Archive - this option will take the currently opened encrypted archive and remove the encryption without creating a new non-encrypted +archive. The opened(same) archive will remain in the original location from where it was opened and will no longer be encrypted.
New Archive - this option will keep the original archive in the original location and create a new and non-encrypted archive in the location of your choice with the same files as in the encrypted archive opened in PowerArchiver. You may type in the location into the field next to the New Archive selection or click the browse button on the right side and select a location where the non-encrypted archive will be saved.
After you select whether to decrypt the same archive or create a new decrypted one with the same files, Same or New Archive, click OK and the decryption process will begin.