[MENTION=467]Reynard[/MENTION]
As a rule of thumb, I'd add +2 to Hit and Damage rolls, and add a 1d6 to the fire damage from its attacks. I think the CR rules in the DMG would only indicate a +1 to attacks, but I'd go a little more.
Raise its AC to 21, which won't make a huge difference, but will turn a few hits into misses
Raise the DC for Frightful Presence, Breath Weapon, and Wing Attack by 1 (Wis DC 20, Dex DC 22, Dex DC 23, respectively).
I'd also add a good amount of HP...probably about 100. Or give it some other means to help mitigate damage. A simple Potion of Invulnerability would allow it to take half damage from all of the PC attacks for 10 rounds.
These increases put it somewhere between the CR 17 Adult and the CR 21 Ancient.
However, I think the key to this will be to increase the number of Legendary Actions and uses of Legendary Resistance. I think you could double each, at least. And as [MENTION=6785438]Warmaster Horus[/MENTION] suggested, maybe create a couple of unique Legendary Actions that you think would work well given the encounter design, terrain, party, etc.
Maybe a "Tail Sweep" that allows a tail attack against up to 3 adjacent enemies, that also forces a save or fall prone.
Maybe a "Snatch and Grab" that allows the Dragon to move up to half its movement without provoking, and make a grapple against one enemy. If grappled, the enemy is held in the dragon's claws and needs to succeed on a Dex-Acrobatics or Str-Athletics check to escape.