If you are not having Sharepoint 15 hive access you can change folder image in the following way.

Step 1:

upload the new folder icon in style library

Step 2:

Create a test document say ex:TestLib library and In that create few folders
1) Test1
2) Test 2
3) Test3

Now create a new Sharepoint page and insert the TestLib webpart. In the same page create a script editor webpart and add the below code.

Now you can see the folder icon changed with new image you placed in style library as mentioned in Step 1

<style type=”text/css”>
.ms-vb-icon > IMG, .ms-vb-icon > a > IMG {
width: 40px; /* increase image width to 40 px */
content:url(“https://xyz/sites/abc/Style%20Library/Images/Places-folder-red-icon.png “);