I’m preparing to run a game that’s going to start off with princes of the apocalypse and later meld into baldurs gate descent into Avernus. Assuming the characters will be around levels 10-14!how would I go about scaling the creatures up or just ramping up the combat encounters?

Here are some options:

• Give the opponents max HP or min HP instead of average HP
• Increase or decrease the damage that the opponents do. Instead of 1d6 + 2, either make it 1d6 or 1d6 + 4.
• Increase or decrease the number of opponents
• Add a second wave of opponents who come in when half of the first wave has been defeated 
• Give the opponents special abilities. The ability to cast a 1st or 2nd level spell once a day for example. 

One other thing. My NPCs use any and all magic items that they have. Magic items don’t sit in a treasure room, they’re in the hands of the opponents who the PCs are facing and they use them! That +1 weapon that a Necromancer has? Yeah, it’s being used by a skeleton against the PCs. 

In Baldur's Gate: Descent into Avernus, it is very easy to convert the adventure into a higher level adventure. Here are a few tips:

1. Skip the introduction chapter. You don't need to have them fight against the cult of the Dead Three, just skip it. If you want to skip that, you can just have them go to Elturel for some reason, and then get stuck there as it is dragged into Avernus, and they have to figure out what to do from there. The only loose threads connected to this is Thavius Kreeg with the Gargauth in the shield, and Lulu. What I recommend to solve this is that Lulu never left Avernus, and saw Elturel appear out of the sky, and flew up there to help. Thavius Kreeg had the Shield, and Zariel dragged him down to Avernus with the contract as Elturel was taken.
2. The encounters in Avernus are fairly easy for level 10-14 characters to win against. The easy thing to do is just to increase the rank of the demons/devils they'll be fighting. If it was a group of bearded devils before, maybe turn them into barbed devils.
3. The more powerful encounters will seem easier to beat, if they want to take on Arkhan the Cruel, along with his Tortle Death Cleric, and Minotaur Bodyguard, they are more likely to be able to do so, but instead of just making the evil creatures that can possibly help them physically intimidating, really drive in the idea that if they kill those NPC's, they will be angering gods, and archdevils, and demon lords. This should persuade them not to go on a Devil murder-spree, and not kill everything in the Nine Hells that they can see.
4. Make the villains smarter. If the fight is against a group of flying devils, instead of flying down to melee against the barbarian, have them fly up, and attack them with ranged weapons. This will make the fights harder, and you might want to make them more likely to run away as well. Don't make the villains harder, make them smarter.

Thank you very much!

No problem.

I am currently running a Descent into Avernus campaign that will likely go higher than 13th level, and will delve deep into the aftermath of their actions in both the Material Plane and the Nine Hells.

If you need any other advice, I'm happy to help.