replace string in field
This replacement can be done with:
UPDATE tableName SET fieldName =
     regexp_replace(fieldName, '\yreports\y', 'reports_test', 'g');

From the documentation:

\y matches only at the beginning or end of a word

The last argument 'g' is meant to replace all occurrences, otherwise it would replace only the first found.
You may also add a WHERE clause to limit the rows updated to those that match:
UPDATE tableName SET fieldName =
     regexp_replace(fieldName, '\yreports\y', 'reports_test', 'g')
WHERE fieldName ~ '\yreports\y';