Hello David, and welcome to the Community!
Do you have the "Manage Sprints" permission in your Jira project? My guess is that you currently do not have it which is why you're unable to change the sprint dates. Your Jira and/or project administrator (if it's not you) can make this change.
Here's some more information about the Manage Sprints permission.
All that needs to be done is to check the permission scheme associated to the project, and determine which groups or project roles have the Manage Sprints permission. Then, make sure your user is a part of one of those groups or project roles.
Hope this helps
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