I'm dming the campaign descend into avernus, and I'm reading through the stat block of zariel, and I notice a spell like fireball is being able to be casted at will, 'ofcourse original 3rd level, but would it be possible they cast it for example at 4th level, would this still not require a spell slot? Or can they cast the spell at a higher level, but then not anymore at will but at the cost of a spell slot?

So for short how does up casting work with at will spells that ain't cantrips

So if I'm looking at the right stat block: 

Innate Spellcasting. Zariel's innate spellcasting ability is Charisma (spell save DC 26). She can innately cast the following spells, requiring no material components :

At will: alter self (can become Medium when changing her appearance), detect evil and good, fireball, invisibility (self only), wall of fire
3/day each: blade barrier, dispel evil and good, finger of death

The way I would interpret this is that no, Zariel cannot upcast. He doesn't seem to have typical player spell slots, just at will spells and the spells in a 3/day category. 

As DM you can always have him do it anyway if you want but as written I don't think upcasting is intended. 

Neither Mords nor Zariel isn't described as a "level anything" spell caster with spell slots.  Her innate magic is as is.  It's not like she can swap out her innate for some of her more powerful spells as written. I've never seen any official guidance or ruling advising taking a monster's limited spells and using those spells' levels to cast at will spells (say give up finger of death for a big fireball). 

Not sure why you'd want to upgrade her fireball, she's got a pretty potent arsenal as is.  That said, if to make her suitably challenging to your parties, there's nothing wrong with you as the DM boosting her so that her fireball is cast at 4th level, it just wouldn't reflect the CR she's written at.  

A lot of folks feel DitA is beyond hard as written.  I'm not as convinced, I think the adventure throws a lot of monsters at you that will trounce a party in a straight up fight ... as a means to coax the adventurers into "dealing with devils" instead of just fighting everything.  So I don't know if you have some simulation where Zariel is coming up short against your party, but she seems to me to be a more than adequate antagonist for around 5 12-13th level PCs.  But again, you can always ramp up or scale down to suit what you're trying to accomplish or see the PCs accomplish in the challenge.

i was just curious if it was possible in mechanic wise, especialy since my party dealt with ease with every encounter so far besides the master of souls and it was fireball that made that encounter so hard.

sounds fair, what if zariel had spellslots for higher level spells? would that change it? does zariel need to use spell slots for higher level spells of his at will spells?

So, if you check out other "monster" stat blocks in DitA, you'll notice some are listed as "x level spell caster" (like the Master of Souls) and then basically gives you their spell slots and prepared spells.  Maybe someone has, but I don't know how you'd translate the ability to cast fireball limitlessly into a leveled spell caster's tier.  Fireball is basically one of Zariel's cantrips in terms of availability to her.

It's really not something you'd need the justification of an official rule for. The official encounters are a one-size-fits-all concept, but as with all such things this size fits some better than others. If, for whatever reason, your players waltz through every encounter you're absolutely free to make them a little more challenging - and arguably it'd be better for the campaign.