upgrade: add upgrade tidb using tiup offline (#2766)

TOC.md

@@ -50,6 +50,7 @@
 + Maintain
   + Upgrade
     + [Use TiUP (Recommended)](/upgrade-tidb-using-tiup.md)
+    + [Use TiUP Offline (Recommended)](/upgrade-tidb-using-tiup-offline.md)
     + [Use TiDB Operator](https://pingcap.com/docs/tidb-in-kubernetes/stable/upgrade-a-tidb-cluster/)
     + [Use TiDB Ansible](/upgrade-tidb-using-ansible.md)
   + Scale

upgrade-tidb-using-tiup-offline.md

@@ -0,0 +1,30 @@
+---
+title: Upgrade TiDB Using TiUP Offline Mirror
+summary: Learn how to upgrade TiDB using the TiUP offline mirror.
+category: how-to
+---
+
+# Upgrade TiDB Using TiUP Offline Mirror
+
+This document describes how to upgrade TiDB using TiUP offline mirror. The upgrade steps are as follows.
+
+## Update TiUP offline mirror
+
+1. To update the local TiUP offline mirror, refer to Step 1 and Step 2 in [Deploy a TiDB Cluster Offline Using TiUP](/production-offline-deployment-using-tiup.md) to download and deploy the new version of the TiUP offline mirror.
+
+    After you execute `local_install.sh`, TiUP completes the overwrite and upgrade.
+
+2. Redeclare the global environment variables according to the result of `local_install.sh`. Set `TIUP_MIRRORS` to `/path/to/mirror`, the location of the offline mirror package printed by `local_install.sh`.
+
+    {{< copyable "shell-regular" >}}
+
+    ```bash
+    source .bash_profile
+    export TIUP_MIRRORS=/path/to/mirror
+    ```
+
+    The offline mirror is successfully updated. After the overwrite, if an error is reported when TiUP is running, it might be that `manifest` is not updated. To fix this, you can execute `rm -rf ~/.tiup/manifests` before using TiUP.
+
+## Upgrade TiDB cluster
+
+After the local mirror is updated, refer to [Upgrade TiDB Using TiUP](/upgrade-tidb-using-tiup.md) to upgrade the TiDB cluster.