Document one-line upgrade via install.sh re-run

Clarify that re-running the curl installer upgrades in place, and detect
an existing install so the script prints an Updating message.

Co-authored-by: Frank Vitrano <WHC2006@users.noreply.github.com>

install.sh

@@ -22,9 +22,11 @@ for arg in "$@"; do
 		cat <<'EOF'
 Usage: install.sh [--user] [--uninstall]
 
-  (default)  Install system-wide to /usr/local (needs sudo)
-  --user     Install to ~/.local (no sudo for the link)
+  (default)  Install or upgrade system-wide to /usr/local (needs sudo)
+  --user     Install or upgrade to ~/.local (no sudo for the link)
   --uninstall  Remove installed miaur
+
+Re-running this script upgrades an existing install in place.
 EOF
 		exit 0
 		;;
@@ -108,7 +110,11 @@ do_install() {
 
 	[[ -f $SRC/miaur && -d $SRC/lib ]] || die "incomplete source tree: $SRC"
 
-	echo "==> Installing to ${SHARE_DIR}"
+	if [[ -e $BIN_LINK || -d $SHARE_DIR ]]; then
+		echo "==> Updating existing install at ${SHARE_DIR}"
+	else
+		echo "==> Installing to ${SHARE_DIR}"
+	fi
 	$SUDO mkdir -p "$SHARE_DIR" "${PREFIX}/bin"
 	$SUDO rm -rf "$SHARE_DIR"
 	$SUDO mkdir -p "$SHARE_DIR"