diff --git a/src/Helpers.hs b/src/Helpers.hs
index 21fa267..9f48516 100644
--- a/src/Helpers.hs
+++ b/src/Helpers.hs
@@ -27,7 +27,8 @@ module Helpers (
   urlToText,
   makeHttps,
   editHost,
-  dropParam
+  dropParam,
+  editParam
   ) where
 
 import qualified Data.List as L
@@ -96,6 +97,27 @@ dropParam pName url = let
   params' = filter (\p -> fst p /= pName) $ params url
   in url { params = params' }
 
+-- | Edit a parameter
+editParam
+  :: String
+  -- ^ the parameter name
+  -> (String -> Maybe String)
+  -- ^ the transformation function
+  -> Url
+  -- ^ the 'Url' being edited
+  -> Maybe Url
+editParam pName f url = do
+  params' <- mapM
+    ( \case
+      (name, Just val) -> if name == pName
+        then do
+          val' <- f val
+          Just (name, Just val')
+        else Just (name, Just val)
+      x -> Just x
+    ) $ params url
+  Just url { params = params' }
+
 subToUrl :: T.Text -> Maybe Url
 subToUrl text = case T.splitOn "://" text of
   [protT, raw] -> do
diff --git a/test/HelpersSpec.hs b/test/HelpersSpec.hs
index 646908c..23e59bb 100644
--- a/test/HelpersSpec.hs
+++ b/test/HelpersSpec.hs
@@ -36,6 +36,7 @@ spec = do
   makeHttpsSpec
   editHostSpec
   dropParamSpec
+  editParamSpec
 
 textToUrlSpec :: Spec
 textToUrlSpec = describe "textToUrl" $ mapM_
@@ -123,6 +124,31 @@ dropParamSpec = describe "dropParam" $ mapM_
     withVal    = simpleUrl { params = [("b", Nothing)] }
     withoutVal = simpleUrl { params = [("a", Just "1")] }
 
+editParamSpec :: Spec
+editParamSpec = describe "editParam" $ mapM_
+  ( \(desc, pName, f, expected) -> context desc $
+    it ("should be " ++ show expected) $
+      editParam pName f url `shouldBe` expected
+  )
+
+  --  description,     param name, function,       expected
+  [ ( "reverse",       "a",        Just . reverse, Just reversed )
+  , ( "fail",          "a",        const Nothing,  Nothing       )
+  , ( "reverse empty", "c",        Just . reverse, Just url      )
+  , ( "fail empty",    "c",        const Nothing,  Just url      )
+  , ( "no reverse",    "d",        Just . reverse, Just url      )
+  , ( "no fail",       "d",        const Nothing,  Just url      )
+  ]
+
+  where
+    url      = simpleUrl { params = mkParams "foo" }
+    reversed = simpleUrl { params = mkParams "oof" }
+    mkParams val =
+      [ ( "a", Just val   )
+      , ( "b", Just "bar" )
+      , ( "c", Nothing    )
+      ]
+
 simpleTxt :: T.Text
 simpleTxt = "http://example.com/"