There are two options:
Option 1 - You can enable the job board accounts which will prompt the candidate to create a username and password when applying for your job(s) and they can edit their application. You can do this by going to Setup-->Job Boards-->Edit Settings [for Public Job Board] and setting these two items to "Yes":
Any current candidate will need to create a new account using their existing email address and then choose "Forgot password" to create a password. Then their username will be their email address.
Option 2 - You can go to the candidates "Answer" tab and choose the edit icon for the questionnaire you would like them to edit here (as an example):
This will open up a new web tab with a unique url for the candidate here:
Copy and paste that url in an email to ask the candidate to update the questionnaire.
Please note that every candidate will have a unique url so this should only be shared with the intended candidate.