I want to only show the excerpts, not the entire content,
This question has been asked and answered in respect of the twenty eleven theme.
Matisse theme is different. where can i find the files to alter content to excerpts
I'm not familiar with the Matisse theme, but start by looking at
loop.php and see if you can find
the_content(); in there somewhere. If so, swap it out for
the_excerpt(); and see if it shows up as expected.
If that's the answer you need, check out the Template Hierarchy in the Codex to find other files that are outputting the full post where you want an excerpt.