in default or standard settings QlikView connects to data sources in read only mode, so you can't modify out of the box.
You could use input fields to modify data within the QV data model only, i.e. this won't change the data source (this is most often used for what-if-analysis). Check the HELP if you want to use input fields.
I assume you want to modify data and write it back to the original data source. You probably want to use input fields in combination with some macros that write your data back to the source.
There are some solutions already discussed here, the appropriate one depends on your needs and settings.
Search the forum for "writing data back to database" or similar.
Hope this helps,