If you fail a saving throw, you can reroll it with a bonus equal to your Fighter level. 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 before a Long Rest starting at level 13 and three times before a Long Rest starting at level 17.
RollQuest ships the D&D 2024 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.