Answer:

1. Check whether the network is connected and whether the server is turned on normally.

2. If it is version 20120316, 20120408, 20120820, or the multilingual version 20120416, you can upgrade it online on the server.

3. If the update still fails, delete the SqlVer.xml file in the server installation directory and then upgrade the database.

4. If some POS work but some don't, it may be that the local office file is not registered. Open Start ---> Run ---> enter regsvr32 msjetoledb40.dll.