Highlight the scale bar and delete it. If you are working with more than one map frame, be sure to select the proper map frame in the table of contents and then insert your new scale bar. When you choose to insert your scale bar, the process will allow you to decide what standard of measurement will be used for the scale bar. Here’s some ESRI based documentation for more help.
Occasionally, dynamic items like scale bars behave oddly. For simple items it’s often easier to delete them and add a new one than it is to “fix” the item itself.
Hope that helps.