Unfortunately there is no ALTER SYNONYM. There is however a feedback in Microsoft Connect asking for this functionality. The reply from Microsoft is not very clear though. They talk about some added functionality in katmai, but I am not able to understand that. Check this link
I think synonyms are vastly under-rated and under-utilized. Consider this scenario. You are running SQL Express with a limit of 4 GB per database. When your DB is almost at 4 GB, just move the larger tables to another DB and create a synonym in the original DB and you have effectively increased your DB size to beyond 4 GB.
Though the answer to the OP's question is NO, just thought I should share this with the community to benefit those of us using SQL express and are stumped with the 4 GB limit.
Raj