Change Dimensions in LayOut

If I do dimensions in LauOut and get an uneven number like 9621 mm - is there a way I can manually change that to 9600 mm

You can double click on the dimension a couple of times to get into the dimensions and edit it to say whatever you want it to say. Personally I’d be looking into why it’s 9621 instead of the expected 9600 and I’d fix that. Editing the dimension will disconnect it from whatever is being dimensioned so if you change that thing later, the dimension won’t keep up with it.

If you want to keep the connection between item and dimension but want to show less spurious accuracy, you could change to cm or metres and reduce the number of decimal points.