Fix data imported in the past

Fix data imported in the past

I recently started with a company that uses ZOHO CRM.  While reviewing the enormous amount of data (leads) in their system, I noticed that there were a LOT of errors in the importing such as a lot of the data was imported into the wrong fields and so its impossible to reduce duplications, etc..  What is the best way to fix all of these errors?

1. Create a report with ALL leads and export into a .csv and then try to manually correct all of the errors (there are over 100,000 leads...)?

2. Is there a process within the system to do this?

Please help if you can.  Thanks!!!