Can’t Change the default theme on WordPress by BitnamI running on AWS

Can you check the permissions of the wp-content and wp-content/themes folders? They should be 755 (owner=write, group=read, public=read).

If, for some reason, the user on your instance that runs the webserver (probably www or www-data) is not the owner of that directory but in the group that owns that directory it should be 775 (owner=write, group=write, public=read)