Squier guitars are very popular among beginning players. Billed as Fender's "value brand," they are cheap and reasonably accurate imitations of the more expensive and usually superior guitars made by their parent company. While no modification will ever make an inferior guitar exactly like the real thing, there are some ways to improve upon the Squier.
Change the tuners. A common complaint about Squier guitars is that they go out of tune easily. A set of locking tuners will help, and should cost less than $100.
Consider a locking tremolo. If locking tuners don't help, if you don't want them or if you find yourself knocking your guitar out of tune with the tremolo bar, you may consider a locking tremolo system. The most popular are those made by the Floyd Rose company. They will keep the strings in tune but do take a bit of getting used to. They also make restringing the guitar more difficult and time-consuming.
Get your guitar set up. If your guitar has poor intonation (when the guitar sounds in tune in some places and out of tune in others), take it to a local guitar shop and have it set up. The luthier (someone who is expert in making, repairing and maintaining guitars) should adjust the intonation as well as the action (the distance between the strings and the fretboard.) Action is a matter of personal preference, but remember that action too low will create an unwanted buzz from the strings vibrating against the fretboard, and action too high can make the guitar difficult to play.
Change the pickups. A popular modification to make to a value-brand guitar is to remove the stock pickups and install better ones. Popular models for the Squier include Lace Sensors and Seymour Duncan Hot Rails. If you're looking to improve the guitar's high-gain sound, you may also consider a stacked humbucker, essentially a humbucker squeezed into the size of a single-coil pickup.
Change the strings. Sometimes poor tone can be fixed by simply changing the strings. If your tone is sounding thin and weak, you may consider using heavier-gauge strings. While harder to play at first, heavy strings will improve your technique and hand strength, and because they send a stronger signal to the pickups, your guitar will sound bigger and fuller. If you switch to heavy strings, have the guitar set up for that gauge. Failure to do so can place extreme stress on the neck and bridge.
Change the pick guard. You may be perfectly happy with the way the guitar sounds, but not with how it looks. A new pick guard is a cheap and easy modification to make, requiring only the removal of a few screws. Choose a pickguard that fits the character you want the guitar to have; dark, bright, whimsical, whatever you wish.