Consider the following configuration of a DatabaseUpdater:

Database to update

This section contains the format and dataset to work on. In the example above we're updating an Esri File Geodatabase.

Parameters

Here we specficy which table we want to update. In this example the table is called "MyFeatureClass"

Match on

This section contains the criteria for which records to update. In this example we're comparing the OBJECTID column with the incoming attribute "MY_OBJECTID". This means that for each incoming feature, the attribute MY_OBJECTID will be compared to the OBJECTID column in "MyFeatureClass". Only matching columns will be updated.

Columns to update

This section describes which columns will be updated, and with which value. In the above example, the column "MyAttributeName" will be updated with the value of the attribute "MyNewValue". This will of course only happen for the records matching the criteria set in the Match on section.

Summary: The above settings will update the contents of the table "MyFeatureClass" in the file geodatabase "test.gdb". For each record in the table where OBJECTID matches the incoming feature attribute MY_OBJECTID, the existing column MyAttributeName will be updated with the value of the incoming feature attribute MyNewValue.

Otherwise my recommendation is to play around with the transformer on a test table, nothing better to learn how it works.

It's also possible to update table contents without using the DatabaseUpdater, here's fairly detailed video that explains how:

https://www.youtube.com/watch?v=VBy8QWBSUyc