By default, only main project managers are able to make changes to a sprint's properties, such as editing members and start/end dates. However, as a project manager, you can change this so that other users become able to edit these properties.
To do this, right-click the sprint and click Allow sprint members to insert, delete or change items. A check will appear next to the option in the context menu. To undo this, and revert to the default settings, you just need to select this option again.