All the segments you have saved are available in the Segments section in the Yandex.Metrica interface. This section shows the number of users in a segment for the past 7 and 30 days, and whether the segment is used for retargeting lists in Yandex.Direct. For more information about retargeting lists, see Yandex.Direct Help.
If you want to edit a segment, click the line for the desired segment. This opens the Sources, Summary report with the selected segmentation condition applied to it. Then click the condition you want to change.