You many want to make a backup of the file before you do any Mass Update.  Here is a brief outline of the steps:

Open the form for the lookup table.
Retrieve all records to be Mass Updated.

Select the Mass Update tools on the Lower Left section.
Go to any field, (DO NOT USE use the Global Code section) and insert your Mass Update programming specs that should look something like this:

IF NOT @ISBLANK(Salary) THEN {
     Salary = Salary * 1.04
     }
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You may want to save the spec for future use:
Close the Editor.
Click on Open Other Specs,  highlight Mass Update. and Click on the SaveTab/Button.  Give the specs a name and Save them.
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Double click on Mass Update on the tree to run the Mass Update routine..

This assumes that the element name is "Salary" and you want to increase the value by 4%.  I will leave you to handle the rounding and decimal places.
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Subsequent edited comment:  I have modified a reference I made to Global Code section.  Erika points out that Global Code will not run on every record in Mass Updates.

Ok, I updated the lookup table using the information you provided. Now, the lookup command won't lookup the revised data.  What am I doing wrong?