To do so, simply make use of the toUpperCase() or toLowerCase() methods. For example:
<input type="text" name="myField" id="myField" size="20" onKeyup="javascript:this.value=this.value.toUpperCase();"> : To Upper <input type="text" name="myField" id="myField" size="20" onKeyup="javascript:this.value=this.value.toLowerCase();"> : To Lower
Below is the working sample:
To Upper To Lower
If you do not wish to run it for each key-up such as in this example, try the onChange event, which will only fire when the user leaves the input field.