For many conferences topics may be set for poster events by the paper authors. Find your poster events on your My Stuff page. These topics may be used by a conference for grouping the posters in the poster sessions. Since they are used for physically grouping the poster events in a session only a single topic is allowed. You can't have a physical poster in two positions at once. Other events may have their topics set as well but poster events are the most general place for a paper presentation. Once poster positions have been assigned topic changes are still allowed but will not result in a change to your poster position.
If you are an author for a poster and the conference has configured topic areas then you can select the topic for your poster. Do this by logging into the website using the email associated with the poster event and then visiting the metadata page for the poster. On the metadata page you can select the topic for your paper from a drop down menu. You can find all your poster event(s) for the conference on the My Stuff page.
The topics list is maintained by the conference staff, changes may be made by the program committee. A limited size list of broad topics is important in order to have a useful grouping of posters.
While you may change your topic at any time, the posters are grouped in a poster session several weeks before the conference, see the Dates information for the conference for this date. Once the groupings are published they will not be recreated so topic changes after this time will no longer be reflected in the session groupings.