You describe to application servers - I am assuming they both perform the same functionality and are load balanced - is this correct? If this correct than you should be able to upgrade each individually with no impact to the end users -
The database will be a more tricky - because it is a centralized database it will require some down time while the database is updated -
Either way key things that need to be done to ensure success are:
Backups
Have a development environmet where you can practice - this can be virtual
