I'd be glad to help, @qiuliang-luo.
You can always delete another user since you are the Master Admin of the company. To do this, just perform the following steps:
For more details about managing your user's access, you can refer to this link: Add, delete, or change user access.
Also, if you downgrade to Simple Start, this will only allow 1 billable user (Master admin). You can check out the following links below for the user and feature plan comparison.
Learn about user types in QuickBooks Online
Compare QuickBooks Online
This should guide you in the right direction. Keep me posted if you need further help with your company users. I'll be here anytime.