Hi, I have a Squier Start HSS and I was thinking about buying a real Fender Stratocaster but the prices are a little too high for me, so now I wanna know how I can upgrade my Squier Strat parts to make it sound better.
electronics, bridge, tuners, maybe the nut. change all those out, and you should have a pretty solid instrument
The first thing to address is always the playability. Many people overlook this, but a proper setup does affect tone because it enables you to play your best.

1.) Neck straight. The trussrod should be adjusted so that the neck is straight, with the fingerboard side ever so slightly concave. The trussrod is not the place to adjust the action (how high the strings are off the fretboard).

2.) Frets level and properly dressed. If not, you can have buzzing issues.

3.) Action adjusted to your taste. On a strat you can individually adjust the saddle heights. if the height at the nut is wrong, you can cut the slot deeper, or replace the nut.

4.) Intonation properly adjusted. This requires some patience on a strat because of the tremolo.

Once you have done these things, you will have a better idea of whether there are further improvements you would like to make. Now you can evaluate whether you need new tuners, bridge, etc, or want new electronics.
A well made and setup Squier can be made to play and sound every bit as good as an American strat.