Once you have created a bar chart, click on the measure or dimension on which you want to sort. (bar chart below without sort)
The selected pill is highlighted in a darker shade, then right click and select sort.
A dialog box opens as below.
Then select field option from the drop down from sort by option. (bar chart after sort below)
Then select the required field (Dimension/Measure) and the aggregation function.
Once you apply the sort. You can see the sort image next to the measure or field on which sort is applied.
You can edit the sort in the same way in above steps.
You can clear sort by right clicking field and click on clear sort option.
Hope this helps you.