Keeping small things aside, in short, you need to wrap your content controls in an Udpate Panel to enable partial rendering of a defined part:
Here you go:
UpdatePanel Control Overview[
^]
UpdatePanel control[
^]
UpdatePanel Class[
^]
All you need to know with sample code is present and explained.
P.S.: You cannot avoid postback. There will be postback but only content part will get refreshed.
In case, you really want to avoid postback, you need to read on
PageMethod[
^].