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>

README.md

@@ -26,6 +26,26 @@ Install for the current user under `~/.local`:
 
 The installer adds the required packages automatically: `base-devel`, `git`, `curl`, and `jq`.
 
+## Upgrade
+
+Already installed (preferred):
+
+```bash
+miaur self-update --noconfirm
+```
+
+One-line reinstall from GitHub (same as install; overwrites the current prefix):
+
+```bash
+bash <(curl -fsSL https://raw.githubusercontent.com/WHC2006/miaur/main/install.sh)
+```
+
+User install:
+
+```bash
+bash <(curl -fsSL https://raw.githubusercontent.com/WHC2006/miaur/main/install.sh) --user
+```
+
 ## Usage
 
 ```bash

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)
+  (default)  Install or upgrade system-wide to /usr/local (needs sudo)
-  --user     Install to ~/.local (no sudo for the link)
+  --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"