I encrypted a form, I have the key but I don't know how to unencrypted the form to see it

Are you planning to decrypt the form submissions? If yes, then please go to the Submissions page and upload the encryption key. Please refer to these guides:

How-to-View-Form-Submissions
How-Can-I-Decrypt-the-Data-from-the-Email-or-Through-the-Submissions-Page.

If you want to turn off the form encryption, please refer to this guide: How-Can-I-Turn-off-the-Encryption-on-My-Form

For more information about form encryption, please do check this guide: Encrypted-Forms-and-How-to-Use-Them

Hope that helps. Thank you.