You cannot with QuickBooks Pro.  In order to do this en masse you need the accountant’s version.  The only option with Pro is to edit transactions one at a time

Resolution for 'change classes issue: 'how to change classes for multiple transactions all at once?' available: Yes.

The change classes 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 change classes issue yourself by looking for a resolution described in the article. If it is a complex change classes issue or you are unable to solve the issue, you may contact us by clicking here or by using other change classes support options.

For more information about resolving issues related to change classes in Quickbooks/QBO or to get support for the issue 'how to change classes for multiple transactions all at once?', click the link below: