You haven't said which but if its one of the OPC assessments, they give you the practice material up front that is quite helpful. You can also search the titles of the assessments on this forum for more info. Having recently done them, best advice is to not overthink it. Make sure you comprehend exactly what the requested task is, and having an understanding of the practice tests will give you good grounding for the real thing.
This is anecdotal, but practicing to "get a better score" in the scope of days or a couple weeks won't work. This is the kind of fundamental testing that attempts to measure speed, accuracy, reasoning, attention, context switching, and spatial awareness which are formed over years. Best thing you can do in the near term is use the provided practice, and be well rested and relaxed.