diff --git a/composer.json b/composer.json
index 9e59dadb4f07ce7c747acba6478585ba22307aed..3d1ecf38b12c2198129d747271d13b5bb66c3c0c 100644
--- a/composer.json
+++ b/composer.json
@@ -61,7 +61,7 @@
         "wpackagist-theme/rusty-grunge": "1.3",
         "wpackagist-theme/simplex": "2.0.1.3",
         "wpackagist-theme/twentyeleven": "4.9",
-        "wpackagist-theme/twentyfifteen": "3.9",
+        "wpackagist-theme/twentyfifteen": "4.0",
         "wpackagist-theme/twentyfourteen": "4.2",
         "wpackagist-theme/twentynineteen": "3.1",
         "wpackagist-theme/twentyseventeen": "3.9",
diff --git a/composer.lock b/composer.lock
index de36db341933a5ab056c753232d0fdc11306f78e..4b6a85dd84ce2be0d257b00b80e7affb01a28b32 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "a5d80fb15113c150fd6fadf3ab693b11",
+    "content-hash": "aabca17cbd54d945e4c4e8797402f98a",
     "packages": [
         {
             "name": "composer/installers",
@@ -1891,15 +1891,15 @@
         },
         {
             "name": "wpackagist-theme/twentyfifteen",
-            "version": "3.9",
+            "version": "4.0",
             "source": {
                 "type": "svn",
                 "url": "https://themes.svn.wordpress.org/twentyfifteen/",
-                "reference": "3.9"
+                "reference": "4.0"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://downloads.wordpress.org/theme/twentyfifteen.3.9.zip"
+                "url": "https://downloads.wordpress.org/theme/twentyfifteen.4.0.zip"
             },
             "require": {
                 "composer/installers": "^1.0 || ^2.0"
@@ -2163,12 +2163,12 @@
     "packages-dev": [],
     "aliases": [],
     "minimum-stability": "stable",
-    "stability-flags": [],
+    "stability-flags": {},
     "prefer-stable": false,
     "prefer-lowest": false,
     "platform": {
         "php": ">=7.1"
     },
-    "platform-dev": [],
-    "plugin-api-version": "2.3.0"
+    "platform-dev": {},
+    "plugin-api-version": "2.6.0"
 }