the naive approach would be to call
Whether or not that is what you need depends on the context of this operation: when implementing a refactoring-like operation that should be written back into the source file you may need a snippet like this:
ASTRewrite rewrite= ASTRewrite.create(ast);
ListRewrite superInterfaces = rewrite.getListRewrite(typeDecl, TypeDeclaration.SUPER_INTERFACE_TYPES_PROPERTY);
If your unfamiliar with JDT's ast rewriting you should have a look at the javadoc of class ASTRewrite for a start.