diff --git a/CHANGELOG b/CHANGELOG
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,7 @@
+2024-09-07
+        * Version bump (4.0). (#532)
+        * Update Op3, Array to support array updates. (#36)
+
 2024-07-07
         * Version bump (3.20). (#522)
         * Update Op2, Struct to support struct field updates. (#520)
diff --git a/copilot-core.cabal b/copilot-core.cabal
--- a/copilot-core.cabal
+++ b/copilot-core.cabal
@@ -1,6 +1,6 @@
 cabal-version:       >=1.10
 name:                copilot-core
-version:             3.20
+version:             4.0
 synopsis:            An intermediate representation for Copilot.
 description:
   Intermediate representation for Copilot.
diff --git a/src/Copilot/Core/Operators.hs b/src/Copilot/Core/Operators.hs
--- a/src/Copilot/Core/Operators.hs
+++ b/src/Copilot/Core/Operators.hs
@@ -106,3 +106,6 @@
 data Op3 a b c d where
   -- Conditional operator.
   Mux :: Type a -> Op3 Bool a a a
+  -- Array operator.
+  UpdateArray :: Type (Array n t) -> Op3 (Array n t) Word32 t (Array n t)
+           -- ^ Update an element of an array.
diff --git a/src/Copilot/Core/Type/Array.hs b/src/Copilot/Core/Type/Array.hs
--- a/src/Copilot/Core/Type/Array.hs
+++ b/src/Copilot/Core/Type/Array.hs
@@ -3,6 +3,7 @@
 {-# LANGUAGE Safe                #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies        #-}
+{-# LANGUAGE TypeOperators       #-}
 
 -- |
 -- Copyright: (c) 2011 National Institute of Aerospace / Galois, Inc.
@@ -14,12 +15,13 @@
     ( Array
     , array
     , arrayElems
+    , arrayUpdate
     )
   where
 
 -- External imports
 import Data.Proxy   (Proxy (..))
-import GHC.TypeLits (KnownNat, Nat, natVal)
+import GHC.TypeLits (KnownNat, Nat, natVal, type(-))
 
 -- | Implementation of an array that uses type literals to store length.
 data Array (n :: Nat) t where
@@ -42,3 +44,21 @@
 -- | Return the elements of an array.
 arrayElems :: Array n a -> [a]
 arrayElems (Array xs) = xs
+
+-- | Update element of array to given element.
+--
+-- PRE: the second argument denotes a valid index in the array.
+arrayUpdate :: Array n a -> Int -> a -> Array n a
+arrayUpdate (Array []) _ _ = error errMsg
+  where
+    errMsg = "copilot-core: arrayUpdate: Attempt to update empty array"
+
+arrayUpdate (Array (x:xs)) 0 y = Array (y:xs)
+
+arrayUpdate (Array (x:xs)) n y =
+    arrayAppend x (arrayUpdate (Array xs) (n - 1) y)
+  where
+    -- | Append to an array while preserving length information at the type
+    -- level.
+    arrayAppend :: a -> Array (n - 1) a -> Array n a
+    arrayAppend x (Array xs) = Array (x:xs)