How to uncheck url path settings in node form alter for existing content?
Here is your answer two ways of doing it 

Method 1:

function module_node_update($node) {
  if($node-&gt;type == "blog"){
        $node-&gt;path['pathauto'] = TRUE;//to be checked
        $node-&gt;path['pathauto'] = FALSE;//To be unchecked
  }
}


or either you can do it in node_presave

Method 2:

function module_node_presave($node){
 if($node-&gt;type == "blog"){
            $node-&gt;path['pathauto'] = TRUE;//to be checked
            $node-&gt;path['pathauto'] = FALSE;//To be unchecked
      }
}


Try and let me know hope it helps 
:)