service pack upgradation on exchange

Very loosely the order should be something like this;

1. Find some time during off peak hours
2. Do a Full System backup.
3. Confirm system prerequisites for the Update and make sure they are all done.
4. Check for and fix any errors which should be fixed prior to running update.
5.  Repeat steps 2,3, 4 until no errors are present
6.  Download latest Service Pack from Microsoft and confirm checksum.
7.  Install Service Pack
8.  Reboot
9.  Check for errors and if errors exist fix them... if not, happy computing you are done.

Keep in mind that for best results you will want to fix any glaring issues before you run the updates.

Follow change management process. Technically, requirement differ with each Exchange and how your Exchange servers are setup in your environment. Like, if you have server or site resilience than the step might totally different. Always read Microsoft installation instruction document. Before patching any Microsoft server.

For tech step, read my article:
https://www.experts-exchange.com/articles/10389/Steps-for-Upgrading-Exchange-2010-SP1-or-SP2-to-SP3-with-Latest-Rollup.html