How can I make it possible for an employee to change his/her personnal data?
The employee must have been given the necessary rights of access in order to do this.
You can manage these rights under Organisation > Access rights or via Employees > Settings
There are three levels
No access.
The employee does not have access to his/her data.
Read-only.
This is the default setting. Every employee can consult his/her own data, but only the fields that you, as an administrator, have made visible under Company > Personal data settings
Request changes.
The employee can request a change, but only on the fields that you as an administrator have made "Editable".