Great thanks for affirmative reply!
More questions:
1) Where to get latest PRECOMPILED
android? How to know if phone supports it?
2) Any website where such a thing is documented?
If you upgrade the phone using the official firmware update released by your manufacturer, it won't void your warranty.
In some cases, the manufacturers don't release upgrades for some phones, some do.
For example, earlier
Sony used to be notorious for this. They released their flagship(at that time) Xperia X10 with Android 1.6. After a LONG LONG time, upgraded it to 2.1 and then flatly refused to provide further updates (which is why now they had to promise that they will release Android 2.4 for all their 2011 models).
Motorola has also done this with some phones.
Samsung Galaxy S on the other hand, released with 2.1, then Samsung released 2.2, then 2.3.3 and then 2.3.4. So you could upgrade to the latest android version without voiding your warranty.
HTC also provides updates at a decent pace.
There can be two other scenario's. One, your company releases the new android version, but not for your country/region/carrier. In that case you can manually download the firmware and update it. Technically, this would violate your warranty but is very safe because ultimately the firmware has been made and tested for your phone.
The other is where the company doesn't release updates for any region. In that case, if your phone has good developer support, modders download the android source code and release it for your phone. This does void your warranty. And for this, you have to root your phone(kind of like "sudo su" which grants you access to the system partition) this definitely voids your warranty.
Refer to xda-developers.com forum for custom rom's/rooting methods etc. relating to specific models.