Modifying a monster

[Okamigekido]

I'm back again, with another question. This time, I'm wondering about modifying a monsters stats and abilities. My main idea at the moment, is at using a Siren. Yet-the only one I can find, is a CR 10-and my players are only a level 1. This type of monster seems to be the only one to fit the environment, and I think they (the players) would love o fight up against one. I'm just not sure how to edit her abilities and stat blocks to challenge the players without just flat out murdering them.
The group consists of a Paladin, Bard, Ranger, Wizard, and Barbarian. They are going to be fighting her in a large pool of water. Any tips on surrounding affects would be nice to receive as well.

[SuperbDolphin]

Are you sure a Sahuagin Priestess dumbed down just a little wouldn't be the better way to go?

[Lunali]

Found the one that you're probably looking at:

Drop the hp down to appropriate levels, 10 or so. Attack bonus down by 2, stupefying touch down 1d6+3, change the spells to 1/day lower end enchantment, save DC down to 13.

No guarantees, but feels about right.

[JackPhoenix]

What do you expect from the monster?

Also, the only Siren I know for 5e is CR 3, which isn't entirely out of reach of 1st level characters. It's still dangerous, especially with enviromental advantage, but not impossible.

[Wryte]

I'd probably start with the Bard NPC from Volo's Guide (CR 2) and give it a swim speed and water breathing.

[Okamigekido]

I understand that the monster is a much higher challenge rating than the players themselves, I'm just trying to see if there is some sort of balanced way to down their CR FOR the players. I definitely want to use the siren (or perhaps use the siren, and take stats from a simpler monster) And, it being some sort of boss, would be likable. (Sorry if I'm not explaining the idea very well)

[DrKerosene]

I have no idea where the CR 10 Siren that you are referring to can be found.

I would just use a Harpy and modify it from there.

I know there is a Neirid and a Kelpie, but those are still no were near CR 10.