Well, it depends on the creature, really. If it says a creature casts "As an X-level Class", then it's fixed at that level, even if they're advanced by other means. If they do advance, this is specifically called out somewhere. Dragons, for example, have a rule for their caster level equaling their hit dice.
Caster level = hit dice is only for the dragon spell-like abilities, not their actual spellcasting. The OP wants to know about actual spellcasting, not spell-like abilities. As for that, it is covered in the rules about adding class levels to monsters.
Next, add the class levels to the monster, making all of the necessary additions to its HD, hit points, BAB, CMB, CMD, feats, skills, spells, and class features. If the creature possesses class features (such as spellcasting or sneak attack) for the class that is being added, these abilities stack. This functions just like adding class levels to a character without racial Hit Dice.
So if a creature that casts as a 7th level sorcerer takes an actual level in sorcerer, it would now cast as a 8th level sorcerer (but have the class features of a 1st level sorcerer). If the same creature instead took a level in cleric, that wouldn't advance his sorcerer casting at all. He would therefor cast spells as a 7th level sorcerer and a 1st level cleric.
hum thank you,but whet if he add a monster hd advancement instead of an actual class i guess her spellcasting also increases right?
Unfortunately, that is less clear. Pathfinder removed the notation from monsters that said if they advanced by HD or class levels. Generally, additional hit dice are for bigger/stronger than versions, not as something gained from experience. Spell casting monsters (actual casting, not spell-like) usually advanced by adding class levels, not HD.
Adding racial Hit Dice to a monster is a similar process to building a monster from scratch. As additional Hit Dice are added, other abilities increase in power as well. Additional Hit Dice usually results in better attack bonuses, saves, hit points, and skills, as well as more feats. It can also include additional spellcasting capability and other powers.
However, it never actually says how to include that. The majority of monster spellcasting abilities (both actual and spell-like) aren't based on hit dice.
The rules don't cover this, but if I advance the HD then I am advancing the spellcasting too. You just have to know how to balance it vs your party as a GM.