To modify fields in database tables, first log in to phpMyAdmin and then follow the below steps:
1. Select the database on the left-hand side which contains the table where you wish to modify the field.
2. Click on the table on the left-hand side that contains the field to modify.
3. Select the field to modify and click the 'Change' icon.
4. Make the changes you require to the field and click 'Save' to complete the modification.