diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index a205f1c9..085bc58d 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -1,6 +1,6 @@
 [versions]
 jacoco = "0.8.7"
-kotlin = "2.0.21"
+kotlin = "2.1.20-Beta2"
 okio = "3.9.1"
 
 [libraries]
diff --git a/koap-core/src/commonTest/kotlin/ByteArrayReaderTest.kt b/koap-core/src/commonTest/kotlin/ByteArrayReaderTest.kt
index 12fea0d8..bb75275a 100644
--- a/koap-core/src/commonTest/kotlin/ByteArrayReaderTest.kt
+++ b/koap-core/src/commonTest/kotlin/ByteArrayReaderTest.kt
@@ -89,5 +89,5 @@ class ByteArrayReaderTest {
 private fun String.parseByteArray() =
     replace(" ", "")
         .chunked(2)
-        .map { it.toUpperCase().toInt(16).toByte() }
+        .map { it.uppercase().toInt(16).toByte() }
         .toByteArray()
diff --git a/webapp/src/main/kotlin/serialization/Hex.kt b/webapp/src/main/kotlin/serialization/Hex.kt
index c13ccfee..e499bb9e 100644
--- a/webapp/src/main/kotlin/serialization/Hex.kt
+++ b/webapp/src/main/kotlin/serialization/Hex.kt
@@ -13,5 +13,5 @@ internal fun ByteArray.hex(
             if (separator.isNotEmpty()) append(separator)
         }
     }
-    return if (lowerCase) hex.toLowerCase() else hex
+    return if (lowerCase) hex.lowercase() else hex
 }