merged update and install page

Signed-off-by: Sam Alba <sam.alba@gmail.com>

doc/install.md

@@ -1,12 +1,15 @@
-# Installing Dagger
+# Installing or upgrading Dagger
 
 ## Summary
- - [Option 1 (Mac OS only): install from Homebrew](#option-1--mac-os-only---install-from-homebrew)
- - [Option 2: install from a shell script](#option-2--install-from-a-shell-script)
- - [Option 3: Manually fetch the latest binary release from Github](#option-3--manually-fetch-the-latest-binary-release-from-github)
- - [Option 4: Compile from source](#option-4--compile-from-source)
 
-## Option 1 (Mac OS only): install from Homebrew
+- [Installing or upgrading Dagger](#installing-or-upgrading-dagger)
+  - [Summary](#summary)
+  - [Option 1 (Mac OS only): install or upgrade from Homebrew](#option-1-mac-os-only-install-or-upgrade-from-homebrew)
+  - [Option 2: install or upgrade from a shell script](#option-2-install-or-upgrade-from-a-shell-script)
+  - [Option 3: Manually fetch the latest binary release from Github](#option-3-manually-fetch-the-latest-binary-release-from-github)
+  - [Option 4: Compile from source](#option-4-compile-from-source)
+
+## Option 1 (Mac OS only): install or upgrade from Homebrew
 
 From your Mac OS terminal, run the following command:
 
@@ -14,9 +17,15 @@ From your Mac OS terminal, run the following command:
 brew install dagger/tap/dagger
 ```
 
+If dagger is already installed, you can upgrade it using this command:
+
+```shell
+brew update; brew upgrade dagger
+```
+
 > :bulb: Verify the installation with `dagger version`.
 
-## Option 2: install from a shell script
+## Option 2: install or upgrade from a shell script
 
 From a terminal, run the following command:
 
@@ -64,4 +73,4 @@ cd dagger; make
 cp ./cmd/dagger/dagger /usr/local/bin
 ```
 
-> :bulb: Verify the installation with `dagger version`.
+> :bulb: Verify the installation with `dagger version`.