What if you accidentally deleted some important apps and can't install them back? What if you accidentally deleted some important apps and can't install them back because Google Play doesn't have them? For example android system, com.android  dun server and com.android.lgsetup wizard (i.e. basically apps that are originally there).
And every few seconds this message will appear:


  "Sorry! The process com.android.phone has stopped unexpectedly. Please
  try again."


And I cannot even go the browser or Task Killer because every time I do, it will force quit. How can I reinstall those apps? I think I cleared all data for those but I did not clear the browser and so why can't I open the browser?

Do I really need to send it for repair or can I fix it?
For common system apps like the ones you've mentioned, there's no way to uninstall or reinstall them: without root, it at all you can disable them (not all of them even). But they are usually not held in the playstore for re-installation. Even if they receive updates via the playstore (like GMail, Maps, and the Playstore app itself), some of them must reside on the /system partition in order to function correctly (the Google Play app is one example for that).
But as the same situation could also apply to &quot;normal apps&quot;, let's take a closer look:
User apps
Though they are usually coming via Google Play, it might well happen they get removed there one day (recent examples are the ad-blockers which got banned). For this, tools like AppMonster come in handy: they enable you to store a copy of the apps .apk file to your SDcard (the Pro version even can do so automatically whenever an app is installed/updated). So in case you have to remove an app for some reason, and want to re-install it later (or even if you want to install the same app on a device without Google account), you can do so by side-loading the .apk. Uninstallation, of course, is easily done either via Settings→Apps, or even via AppMonster itself.
System apps
Here it's a lot different. Without root, you can at maximum disable an app (Settings→Apps, scroll to the app, open its details, press the Disable button) -- if your device is running Android 4.0 or later, that is.
With root, things become a little different: theoretically, you could uninstall everything. A practical issue might be you're rendering your device unusable, if you e.g. uninstall something basic to the system. Also, removing their data might be an issue on re-install. So if you really have need for this, here's my recommendation:

Get ADB installed on your computer (either via the full Android SDK -- or using a minimal installation, see Is there a minimal installation of ADB?
Make a backup of the app you're going to uninstall to be prepared in case something goes wrong. You can use adb backup for this, which backs up the app including its data.
Make a copy of the app's .apk file for a re-install. You can use adb pull for this. The .apk is located in /system/app, so your command could look like adb pull /system/app/Browser.apk . to copy the browser's .apk from the device to the current directory on your computer.
Now that you have two fall-backs, you can try to uninstall the app. Again using ADB, you first call adb shell, then you need to become root (su), and now you can use the pm (package manager) tool to pm uninstall com.package.name. You can also remove the .apk from /system/app.

To re-install the app, you simply copy the .apk back to /system/app. To restore the data, you can use adb restore.
Note that dealing with system apps always bears the risk to render your system unusable. So it's always recommended to make a complete backup (best using Nandroid backup from your custom recovery) prior to such operations -- so in the worst case, you can go back to where you've started.
Dealing with the &quot;The process X has stopped unexpectedly&quot; error
There are several ways to deal with this error. To my knowledge, none of them includes uninstalling a system app.

if the app in question is a user app

delete its cache. If that doesn't help:
delete its data. If that still doesn't help:
delete the app. If that cannot be done due to the app being in a &quot;force-close-loop&quot;:
boot into safe-mode, and delete the app from there.


if the app in question is a system app:

pray you've got a good backup :)
as with the user app: first try delete cache, then data.
nothing goes? Then you've got to do a factory-reset