Increasing CPU Frequency

We are having issues with the board working with 115200 serial baud rate.  Any baud rate slower than 115200 works.  The code appears to not keep up and after the first received character the remaining characters get corrupted.  I suspect the UART hardware buffer is not getting service quickly enough.  I would like to change out the crystal on the board to something faster.  It appears the limitations of the CPU at 3.3V might be around 12MHz.  Where in the arduino code do I specify the new frequency so that all the other internal clock frequencies can adjust accordingly?   Also do you have a part number for the current crystal so I can search for a higher frequency one with the same footprint and capacitance specifications?