The Search and Replace function can be found in the Edit > Find and Replace... menu or by pressing CTRL+F.
The Search component can be used to find all parts of your choice within the layers you select. To do this, simply select the part from the top dropdown list, chose the layers you want to search in and press "Select All".
The Replace component is very useful if you want to change a selection of parts, or all parts of a specific type within certain layers, to something else. Good examples are the changing of 9V track to 12V track, or if you want to change the type/version of a specific roadplate which you have used within your layout.
The Where? section let you specify in where to search the parts. If you choose "In the Current Selection", it will only search the part in the current selection, therefore sub-select all the instances of this part that were in the selection (if you click "select all") or replace only the instances of this part that were in the selection (if you click "replace all") leaving the other instances in the current layer. Moreover if you choose "In the Current Selection", the Find dropdown list contains only the parts in the selection, which is usually much fewer than the whole library. Therefore a quick and convenient way to find or replace one specific part in your layout is: first to click it in your layout to select it, then press CTRL+F to open the Find window, then change the "where?" options to extend the research on all the layers.
If you want to extend the search to one or several layers, you should choose the "In the following Layer(s)" option, which will enable the Brick layer list just under. You can then cherry pick in which layer you want to search. However, be advised that if you pick several layers, you will not be able to select all the parts (because BlueBrick does not support a multi-layer selection yet, this may be open in a future version of BlueBrick), but you can do a replacement in several layers.