About Editing Dimensions
Dimension values can be edited in the graphics window. The model shows the most recently used dimension value. When you double-click the value, a list box shows previously used values.
Dimension properties and display can usually be edited using the Dimension tab. In some cases, you use the Menu Manager to perform these tasks.
When you edit a feature, you can select dimensions and change their values and properties, or you can select a section and isolate the editing to only the section dimensions. You can also isolate the kinds of elements you want to select by filtering your selection.
Negative Dimensions
Creo supports negative dimension values. How the dimensions are displayed, and how the configuration option show_dim_sign affects the display, is determined by the type of dimension
Some dimensions in Creo use references outside the feature and determine the location of the feature. Other dimensions control feature shape.
Most dimensions controlling feature shape are displayed as positive values. If you enter a negative dimension value, geometry is flipped to the opposite side but the dimension value remains positive. For these dimensions you can click 
or type a negative dimension value to flip the feature direction. The configuration option show_dim_sign has no control over this type of dimension.
or type a negative dimension value to flip the feature direction. The configuration option show_dim_sign has no control over this type of dimension.show_dim_sign controls the display of dimension type that determines the location of a feature. For this type of dimension there is no button available to flip the geometry. When set to no, negative values create geometry to the opposite side and dimension values remain positive. When set to yes, the value appears as it is entered. Entering a value with the opposite sign flips the geometry, and with the same sign increases or decreases the value.