Next to dungeons with a solo option you will find a quest giver with the title "Solo/group dungeon switcher". This is the person to talk with to change a dungeon that is normally for groups into a solo dungeon.
Once you talk with the quest giver you will get this screen. Simply accept the quest, but do not complete it. The question mark above the quest givers head will be yellow and this is your indicator that the dungeon is now ready for solo play. Once your done with the dungeon then you can complete the quest and it resets it back to a group dungeon. This is repeatable so you can do it as often as you want.