There are two good ways to "undo" your filters, aside from the back button; it should be noted that all searches are actually unique URLs that can be recreated at any time (except in the case where the API has changed).
The first way to undo your filters by viewing them at the top of the page. As you add a filter, it will show up at the top of the box as a "breadcrumb" (click and drag the slider near arrow 1 and the breadcrumb shows up near arrow 2):
Mousing over the applied filter allows you to click and remove that particular filter:
The second way to "undo" filters is actually a separate feature entirely. When you get down to a single component in the results (or type an exact part number directly in the search box), you might not have breadcrumbs to get rid of in order to "undo" your search. An example search:
Since you don't have the filters to widen, you can do so automatically using "Expand your search"
This will return the filters (which you can then cancel out to expand your search even more) or you can filter to make the "expanded" results more narrow: