If you have recorded the data file with a wrong thermocouple type, there is no direct way to change the scaling. 

The workaround is to use the reference curve. The attachment is an example setup which shows, how is possible to convert from Thermocouple T type to thermocouple K type. 

The setup can be modified, by using different scale factors from the table which can be found here.

NOTE: "Single value based" option was removed from Time reference curve setup from Dewesoft X3 SP12 - Release notes. The substitute is to use Scaling math to configure "Input value" column as "Ref. value" column in old setup.

1. Use the time reference curve, to convert the temperature channel back to voltage. 

2. Then use the second for converting the voltage back to the temperature channel.