search for MIL-HDBK-217 and download it.
calculate the streses for each component, using MIL-217 look up or calculate the failure rates for the environmemnt, temperature and quality level. From the total failure rate calculate the MTBF.
There are calculators to help, but for something that is simple it is probably just as easy doing it manually, unless you need to cover a number of environments or temperatures.