How to round off the value and disabled the textbox inASPNET

Hi I want to round off the value which is in textbox, once value enter in textbox and before cursor moving to next field and also at the same time have to disable the textbox once round off get finished

Eg if i enter 456.667 in textbox means before the cursor move to next field it round off the value 456.667 to 457. Here i tried On value change event function.

Here what is my doubt is here i get the value and also rounded off the value. now my doubt is how to assign the round off value again to same textbox and also disable the textbox once round off. i have less knowledge in asp.net so only i ask this simple question as doubt. i tried my level best to explain the issue.Please any one understand my issue and help me to resolve this problem.