Topic map patcher
Topic map patcher can be used to apply a patch to a topic map. You can create topic map patches using the compare topic maps tool. A patch created by comparing topic maps A and B can be applied to topic map A to produce topic map B. The tool also contains an option to reverse the patch in which case it can be applied to topic map B to produce topic map A. Patch files generally are much smaller than the actual topic maps if the topic maps for the most part haven't changed much.
To start the patcher tool select File > Compare and patch > Apply topic map patch... Choose the topic map source and the patch files. Optionally tick the reverse check box if you wish to apply a reverse patch. If you select the layer stack source option, then changes will be made to the currently selected layer of the layer stack. If you select a single layer, they will be made to that layer. If you select a file then the changes will be written to a new file which has "_patched" appended to the end of the source file name.
Why merge isn't enough
If you already know topic maps, you might ask why you need a patcher while you can always merge topic maps. And very true, you don't need patch operation if your changes are always additive. But when you remove something in topic map, a standard topic map merge can't handle the change. Also, when you change information -- a variant name for example -- the merge operation restores the original variant name and you end up having both original and changed variant name.