You can use dynamo administration UI.
Go to component browser and browse or search the repository for which you want to make a change:
You will see a text area with a button associated. Enter something similar to this:
<update-item item-descriptor="YOUR ITEM DESC" id="ID OF ITEM">
<set-property name="PROPERTY NAME" value="PROPERTY VALUE"/>
Dynamo UI is mend for advance users who understand ATG repository concepts like item descriptor.
Otherwise you can also use BCC to change certain type of items like Catalog items, Profile repository items, Site repository, etc.