Concerning custom default filters, you can do this in two ways.

If you want to manage the filters from the screen they are appearing:

1. Activate the developer mode (see here: https://www.odoo.com/forum/help-1/question/how-can-i-activate-the-developer-mode-83 ).
2. Choose "Manage filters" from the debug dropdown.

If you want to manage all filters from all screens:

1. Navigate to Settings >> Technical >> User Interface >> User Defined Filters.
2. Use the list to locate the filter you want and click it to open and modify.

Concerning system default filters (eg. the "My" filter in sale orders), you should:

1. Activate the developer mode (see here: https://www.odoo.com/forum/help-1/question/how-can-i-activate-the-developer-mode-83 ).
2. Choose "Edit Action" from the debug dropdown.
3. Replace the content of the field "Context Value" by empty curly braces {}