Once you turn on multi-currency you cannot change your home currency.

If your account is less than 60 days old, you CAN purge your data and start all over.  Here is the article on that,

https://community.intuit.com/articles/1282054-how-do-i-delete-all-my-data-and-start-over-from-scratc…

Resolution for 'CAN purge issue: 'How to change my home currency after turning on multi-currency' available: Yes.

The CAN purge issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the CAN purge issue yourself by looking for a resolution described in the article. If it is a complex CAN purge issue or you are unable to solve the issue, you may contact us by clicking here or by using other CAN purge support options.

For more information about resolving issues related to CAN purge in Quickbooks/QBO or to get support for the issue 'How to change my home currency after turning on multi-currency', click the link below: