How to allow users to access their own data to make changes to their submissions

Another problem I am having is how to allow users to access their own data
to make changes to their submissions. Again, if I'm unable to implement
the coding part, then I won't be able to utilize the rest.

This feature is possible already. Please check our user guide on : How to Let Users Update Their Form Submissions at a Later Date

Let us know if you need further assistance.