This operation will affect all transfers that are selected in the main view.  You can select all transfers in the main view by pressing Ctrl-A when the main view has focus.  Then right-click and go to the Trackers sub-menu.

There are a few different ways to match your target tracker(s):

Exact
The target tracker URL must be precisely the same as what is typed.

Wildcard
The ? character matches any single character, the * character matches any number of characters.
For example, *badhost* matches any tracker URL that contains "badhost" at any position.

Contains
The target tracker URL must contain what is typed.

RegEx
Use any Regular Expression to search URLs.  Regular expressions are a more powerful method of string matching.  More information about these can be easily found using a web search engine.

Note that the matching is case-sensitive.

The Test button can be very useful to fine-tune your matches before you proceed to replace or remove.

The final options of Replace or Remove work exactly as you would expect.  The target URL will be replaced in it's entirety by what is typed, or removed completely.