2
create ('clone') a duplicate set of templates in the Template Set Manager (admin>system>templates), select it as the 'Default template set' (admin>system>Preferences >General Settings), save and go edit the code in the blocks you want to change via the admin>system>blocks page.
You can't work on the first 'default' template set that XOOPS generates for you. Those ones are reserved so as you can revert to them as a last resort if you stuff something up on a cloned set...
Make sense?
Never let a man who does not believe something can be done, talk to a man that is doing it.