Giving Score can only be edited on constituent records on a one-off basis. The Giving Score model has taken into consideration hundreds of external demographic and financial characteristics as well as historical giving information from your organization to build the score. It is possible that you may have other personal information about a particular constituent that could influence their score. For example, if John Smith currently has 4 children enrolled in college, the user may want to change John from a VIP to a Fan, so that they can maintain their relationship but not solicit him for a major gift that year. For these reasons, global change or import of the Giving Score is not available.

Alternate solution:
Manually change the Giving Score on each constituent record.