First, you can use single quotes inside your double quoted string. Second, don't use inline event handlers here. Your best bet is to use a real event handler:
function Submin() {
var str=document.Inputs.innerHTML;
document.Inputs.innerHTML="<p style='text-align:center'>The data has been successfully submitted <br /><input type='button' value='Back' ></p>";
document.Inputs.getElementsByTagName("input")[0].onclick = function() {
document.Inputs.innerHTML = newstr;
};
}