1. First of download any image you like for icon
suitable size of image / icon in windows are:
16x16 , 32x32, 48x48, 128x128 , 256x256
note*- If image extension is other then "ico" then convert it into "ico" extension first.
you can use any online convertor "png to ico"
2.Open run > regedit
3. Look for path:
HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Explorer \ FileExts \ .7z \ UserChoice
4. On Right pane , note down the value data for Progid. [in this case progid not exists]
image 1
5. If the UserChoice branch does not exist, navigate to the following branch:
HKEY_CLASSES_ROOT \ .7z
6. In the right-pane, note down the (default) value data. This is the Prog ID for the file extension.
image 2
7. Navigate to the following key:
HKEY_CLASSES_ROOT \ 7z_auto_file\ DefaultIcon
Now if the DefaultIcon key is exists then change the path
If not exists Just create a new key DefaultIcon and enter the path of icon where it resides
Restart your computer so that changes can take effect.