There is not any way to recover or reset the PDF sign signature password once you forgot its current password.
We suggest you delete the signature which you don't know its current password and create a new one since the password protected PDF sign can no longer be used to place signature anymore without inputting its correct password.
To delete it,please follow the steps below (Take Foxit Reader for an example):
Open the user temp folder, somewhere like C:\Users\Administrator\AppData\Roaming\Foxit Software\Foxit Reader\InstaSign. All the PDF sign signatures are saved as pdfs under UserSign folder.
In UserSign folder, delete the one you want to remove.
You could open the signature file to check if it's password protected.