1. 备份旧版数据库

2. 执行新版升级程序

链接：https://domain.name/install/update.php

Update Failed

An error occurred that prevented the update from completing successfully.
Insufficient File Permissions (Help Documentation)
Error: Could not delete WHMCS/vendor/whmcs/whmcs:

这是因为系统升级会把升级包覆盖整个WHMCS安装目录，所以是不可取的。最好是数据库升级和代码直接覆盖。