Change through textbox!!!!

[bcm]

I have created a project of reading app.config configuration file and display it in textboxes
But now I want that when ever the values are displayed in textboxesm, if I change it in textbox it should be simultaneously changed in app.config also.
How can I do that in VB.NET2005???
Please help!!!!
This is my app.config:
<applicationSettings>
<WindowsApplication1.My.MySettings>
<setting name="Astrology" serializeAs="String">
<value>6:00</value>
</setting>
<setting name="Cricket" serializeAs="String">
<value>7:00</value>
</setting>
<setting name="Foreigncurrency" serializeAs="String">
<value>8:00</value>
</setting>
<setting name="Jobs" serializeAs="String">
<value>9:00</value>
</setting>
</WindowsApplication1.My.MySettings>
</applicationSettings>
I have displayed the values 6:00,7:00,8:00,9:00 in textbox and I want that if I change this time in textbox the change should occur in app.config file also.
Please soon!!!