Hi,I would like to deactivate Multicurrency management in order to downgrade my package to Quickbook Start Plus.

Hello -netvisor-uk

Thanks for joining the Community.

QuickBooks Online Simple Start doesn’t support multicurrency. If you currency have this enabled, it's not possible to switch off, meaning you won't be able to downgrade. In this case, the option would be to cancel your subscription and create a new account. 

@-netvisor-uk 

You can signup for a new QBO account and purchase a 3rd party service to migrate data from the old account. Another option, use a migration tool.

https://get.transactionpro.com/qbo