You can modify the existing data loaded in Qlikview. As in Qlikview also we prepare a data model with respective to the data provided/requirements. So transformations are almost necessary for each qvw.
And there are many different ways to do it. Of course depending on what you trying to do.
It is always better to do all the transformations using SQL in DB as it is much faster than Qlikview. However there are few things which end user wants in Qlikview so there you need to apply few functionality.
JOIN
CONCATENATE
QUALIFY
ALIAS
APPLY MAP
RESIDENT LOAD
SET/LET
MASTER CALENDAR
IF CONDITIONS
INLINE LOAD
and many more things can be done but the above are commonly used.
Go through the developers guide for Qlikview and you'll understand it much better.
Search for 'Developers Guide' in Qlik community and you'll find many links/documents for the same.