I currently have an Android project in Eclipse.
I created it with a target build of 1.5 (sdk 3).
Now I want to change it so that it has a minSdk of 3 and tar
Right click the project and click "Properties". Then select "Android" from the tree on the left. You can then select the target version on the right.
(Note as per the popular comment below, make sure your properties, classpath and project files are writable otherwise it won't work)