rigradio I have updated the Joomla write on the fly, but couldn't find for the template & plugin.
You may download and install latest version of plugins and templates following methods as outlined here
The same will override the previous version by default however Would always suggest to always take backup of your site before performing aforesaid steps, in case rollback required whilst facing any issues e.t.c
Also, This guide can also be referred to being part of core joomla feature
Overriding technqiue (Primarily used to avoid changes from getting lost)
1) For making changes in css codes, You may use css override option which would intact the specific file along with changes while perfoming update
2) Some resources can be followed for creating template overrides for specific extensions due to reasons stated in the tutorials :
https://docs.joomla.org/How_to_override_the_output_from_the_Joomla!_core
https://docs.joomla.org/J3.x:How_to_use_the_Template_Manager
For updating JA Extensions and Templates, You can always use JA Extension Manager which also allows for several other features apart from solely upgrading for which upgrade steps are outlined here
Upgrade steps:
View the comparison between versions at JoomlArt Version Updates and replace the affected files, if you have not customized the affected files.
Recommended : Use JA Extensions Manager Component for upgrade. Watch video for how to upgrade using JAEM.
