Sorry, there is no way to change the chain ID in Chimera.  You would need to text-edit A to B in the PDB file before opening it.  (Or, if you're fetching the structure from the PDB, write it out in PDB format, edit the file, and read it back in.)

Depending on what you are trying to do, there may be some way of doing it without changing chain ID.

Best,

Elaine