I have an existing farm that is configured with SP 2013 SP1 + Dec 2014 CU. I want to upgrade this farm with the latest March 2015 CU.  Any specific instructions that I can follow ?  My farm is consisting of 2 App servers, 2 Web front ends and Search is configured as well.  Any help is highly apprec

Thanks for the quick responses. Please note that the earlier DEC CU I have applied is directly while installing the SP itself (including the CU into Product bits by copying the extracted files from DEC CU into UPDATES folder) and installing SP. The question about installing March CU as an update install because I am reading online saying it takes about 5 +hours and is very slow.  Is the same story applicable to March CU as well? 

Also, one more question about adding the CU to the Products set up bits (in UPDATES folder) -> do  I need to copy all files (basically the extracted files are from all language related) or just ENG specific files only is enough ?

Use the script in this blog post to speed up binary installations of any SharePoint 2013 patch:

http://blogs.msdn.com/b/russmax/archive/2013/04/01/why-sharepoint-2013-cumulative-update-takes-5-hours-to-install.aspx

You no longer need to use the Updates folder, just drop the exe (+cab file(s) if applicable) in the same folder as the PowerShell script in the above blog post. Run the PowerShell script and it will automatically install that patch. Only Service Packs are language-specific. All other updates (CU, PU, hotfixes) have all languages.

Don't forget you need to run the Config Wizard after installing any bits.

Thanks very much Trevor.  I am going through the script and will try it.

It seems to be my question on copying the CU files into UPDATES folder  -> What I actually want to get clarified is - for a fresh install of SP2013 SP1 RTM product; what I do is I include the CU extracted bits into the UPDATES folder in the set up bits of RTM - which will take care of installing the RTM + with the CU.(whereas The above script / your answer addresses installing the CU on an existing farm). So, in this case, is just copying the .EXE + 2 .cabs that came with March CU is enough into UPDATES folder ?  Because I am advised to copy the extracted files not just the 3 files (exe, cab).  The problem with this is, the extracted files consist of about 6 GB with 1482 files (most of them are per language specific).

No, what you would do is extract the March CU, e.g.:

march_cu_exe_name.exe /extract:C:\marchcu_bits

This would extract the CU to C:\marchcu_bits. You would then copy all of those bits into the Updates folder.

HI Krushi,You need to extract the file and All the files and the script should be in one folder only to run succesfully.Once the patch is Done you need to run the PSconfig.As Trevor said,All the latest CU's contain the previous CU+hot fix that are released prior to this CU.