Delete data from custom fields of tables for upgrading

Hello Experts,

I have to replace tables in client database for upgrading purpose, but client database have many custom fields in tables and these fields also have data. When i try to replace these objects with standard objects i get error message due to custom fields data.

Now I have to delete all custom fields data from tables and than I will able to replace the tables one by one.

Is there any possible way I can select all conflicting tables than I select all custom fields after that I delete all data from these custom fields ?