You can improve certain aspects of an underperforming guitar by upgrading certain hardware components. In some cases, if not all, a guitar that is otherwise solid in its body and neck, and which has a playing feel that you enjoy, can only be made better by the addition of, for example, a better bridge and/or tailpiece, improved tuners, a new nut, and so on.
Going top-notch with everything, though, may add up to more than your guitar is worth, so in some cases you’re better off just selling what you’ve got and looking for a new (or new used) guitar that was equipped with better parts in the first place.
If you do want to upgrade any or all of your hardware, sticking with retro-fit components, which are precise matches mounting-wise for the original equipment on your guitar, often makes modifications possible for you to do yourself with just a few basic tools.
Be aware, however, that bridge saddles often have to be slotted, a precision job that requires the correct files, and setting up a guitar properly after a hardware change so that it plays its best often requires some advanced skill and previous experience.
You may, for example, improve the tone and function of a relatively cheapo offshore-made Strat-style vibrato by upgrading to one of several high-quality replacements available.
The better of these cost upwards of $135 at the time of writing, not including any labor required to install it if you can’t do the work yourself, which may represent a substantial proportion of the total cost of a budget or entry-level electric guitar that you intend to put it on.
Similarly, the better fully adjustable wraparound replacement bridges (with individually intonatable saddles) can sell for anywhere from $80 to $139, so it’s a big investment unless you’re looking to upgrade a high-end guitar that will benefit from such a part. That said, if you shop around you can generally find good functioning replacement components for a lot less, many of which may still be upgrades on entry-level instruments.
If you’re looking to improve the return-to-pitch efficiency of a vibrato that you use frequently, a roller-saddle bridge (with saddles that roll to transport the strings back and forth during vibrato use) and a set of locking tuners can be had for a price that is well worth the reduction in retuning frustration.