The default sort option for your search results is the Received Date of the document.
You may change the sort option here.
However, if your search returns more than one page of results, please be aware that each time a new page of results is loaded, the sort order of the results will switch between ascending (A to Z or 0 to 9) and descending (Z to A or 9 to 0).
Therefore, to read your results in the correct order across multiple pages, you'll read the first page of results top to bottom, the second page bottom to top, the third page top to bottom, the fourth page bottom to top, etc.
You can also click on the column headings of your search results to initiate sorting.
However, please note that, with the exception of Received Date, clicking the column headings will only sort the results on your current page.
If your search results are spread across two or more pages, the results on the other page(s) will not be affected when you click the column headings, unless it's Received Date.
Searches initiated using the Sort By feature will have their results sorted across all pages, but those search results will be subject to the issues regarding sort order outlined in the preceding paragraph.