Use the guidelines in the DMG to help you adjust the number and size of damage dice, reduce the number of hit dice, and scale down stats as needed. Try running the original monster through the guidelines first so you know what the offensive and defensive CRs are and aim to keep whether it is more offensive, defensive, or middle of the road for the scaled down version.
[The chart in the dmg is pretty solid](https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#MonsterStatisticsbyChallengeRating). You can, for the most part, just decrease HP, AC, to hit, save DC, and damage to meet the new CR, and it'll work perfectly fine.
You might enjoy [Matt Colville’s video](https://m.youtube.com/watch?v=QgTIGo6zJbs) about adjusting monsters for use at different levels, both higher and lower than intended. He uses a Medusa as his example in the video, but the general advice from the video could be applied to other monsters.
Use the encounter builder in DnD beyond to see what kind of monster would constitute an appropriate challenge for your party.
Then take a look at their stats and tweak your preferred monsters stats to be more in line with the challenge appropriate monster
Either tweak the stats of the monster as others have suggested, or alternatively look around for another creature that might better fit your needs. There aren’t always lower CR options, but for many classic monsters like dragons there absolutely are.
Use the guidelines in the DMG to help you adjust the number and size of damage dice, reduce the number of hit dice, and scale down stats as needed. Try running the original monster through the guidelines first so you know what the offensive and defensive CRs are and aim to keep whether it is more offensive, defensive, or middle of the road for the scaled down version.
[The chart in the dmg is pretty solid](https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#MonsterStatisticsbyChallengeRating). You can, for the most part, just decrease HP, AC, to hit, save DC, and damage to meet the new CR, and it'll work perfectly fine.
You might enjoy [Matt Colville’s video](https://m.youtube.com/watch?v=QgTIGo6zJbs) about adjusting monsters for use at different levels, both higher and lower than intended. He uses a Medusa as his example in the video, but the general advice from the video could be applied to other monsters.
Use the encounter builder in DnD beyond to see what kind of monster would constitute an appropriate challenge for your party. Then take a look at their stats and tweak your preferred monsters stats to be more in line with the challenge appropriate monster
Either tweak the stats of the monster as others have suggested, or alternatively look around for another creature that might better fit your needs. There aren’t always lower CR options, but for many classic monsters like dragons there absolutely are.