Yes, there is no direct way to modify 'Smart Albums'
As a workaround, you can use following steps:
1) Select your 'Smart Album'
2) From 'Options' drop down (present near 'Show All button), select modify 'Search 3) Criteria'. 'Find by Details (Metadata)' dialog will open.
4) Change anything that you want to change and save it again as a 'Smart Album'