At the top of the contacts form you'll spot an input field under the column name.

For example, Surname, Phone, Group.

Start typing in the input field.

For the Opt-out field,you need to type true or false to filter. 

To delete, export/download or do an advanced validation for a set of filtered contacts

Create a filtered set of contacts by searching for a group, opt-out,phone etc. 

• Click the 'Select All' box to select all the filtered contacts.
  
• Click the Download icon to download all to excel (you'll be prompted to download)
• Click the 'Trash Can' to delete the selected list (you'll be prompted to delete)
• Click the ? to do an advanced validation (you won't be prompted)