Here are the instructions on how to regenerate new API keys for the PrestaShop PayPal (free version) module:

Step 1.

Log into your PayPal account.

Step 2.

In back office of PrestaShop go to the PayPal module configuration screen.  Click on ‘Get My Paypal Identification info button’

Step 3.

Step 4.

Step 5.

Step 6.

Step 7.

Step 8.

Step 9.

Finally do a PayPal transaction to verify your PayPal module is working correctly.