I have looked at other similar questions on this site but couldn't answer my question. I have a model made of cubes the edges look very sharp (I would put a picture here but I don't have the required rep) is there a simple way to make the edges round?
You could use the Bevel Modifier from the properties panel.
Example with Segments set to 3 and Width 0.1 (red cube) Width 0.3 (blue cube)
stacker's answer answers the question. However, you can also selectively bevel parts of your mesh with Ctrl + B which is often faster than using the modifier if you want to bevel certain parts of your mesh (the modifier has options where you can bevel a group of vertices etc). You can also bevel single vertices with CtrlShift + B.
Also see Most efficient way to round edges and How can I make sharp geometric 3d models look more realistic? for more tips.
If somehow your mesh is all messed up and you're looking for a way to avoid selecting multiple outer edges, the Remesh modifier can sometimes yield adequate results.
This applies in cases when you need a more photorealistic edge to objects - it WILL NOT give you full control of bevelling. Whether it's the best way also depends on the complexity of your shape(s) and how much else is going on in your final render. The more complexity, the harder for the modifier to get it "right".
For increased accuracy, set the voxel size really low (save first, even save as if necessary). This operation better be well thought through though, as it drastically slows things down. Keep in mind your intended end result before going this route.