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If you are referring to still images and
you wish to change the duration in a batch...
The setting only applies to clips added to
the timeline *after* you change the setting.
Tools / Options / Advanced / Picture Duration.
You can change the duration for each clip
manually. Move your pointer around on the
edge of a clip until you see a double red
arrow. Now drag the double red arrow...
you'll see a tooltip that shows the duration
changing.
Or...just redo the project now that you have
changed the duration.