When adding arc dimensions, you should always select the actual arc rather then the center point. Doing so will give you more options for the dimension.
As you can see in the image below, under the leaders tab after selecting a dimension, we can select the Max, Center, or minimum Dimension for the arc. These options are only available if the arc is selected rather then the center point.
So that was one way of changing the dimension after it was created. Below is how it can be done as you are creating dimensions which will save you time.
When placing a dimension the Shift key can change the condition depending on the placement of the mouse, giving you the option to choose where you want to dimension from.