You can use Time Tracking, Story Points or both as tasks duration. To define this, please use the Gantt configuration.
If no estimate is set, the default duration from the configuration is used.
To adjust duration, mouse over the right end of the task bar and once the mouse pointer changes to the resize tool, click and drag left or right to set the desired duration. If you are using Jira Time Tracking, the Original Estimate field value will be updated for issues with no time logged and Remaining for the issue with time logged. If you are using Story Points - the number of story points will be adjusted based on the ratio defined in the configuration.
You can also adjust duration by changing the value of the Jira fields themselves.
If you have both Start and End dates defined, estimates will be ignored and resizing the bar will affect the Finish date instead.