Beginning at 9th level, you can reroll a saving throw that you fail. If you do so, you must use the new roll, and you can't use this feature again until you finish a long rest. You can use this feature twice between long rests starting at 13th level and three times between long rests starting at 17th level.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.