risk-free rates

Hi, I have a question regarding the CAPM model you use in your DCF valuation. In computing the return, do you change the risk-free rate each year or do you leave it constant throughout the explicit period? And if you change it, how do you estimate the Rf for each year? Do you use the yield curve or something similar? Also, in valuating a company outside US, do you subtract the default spread from the yield of the long-term local government bond, assuming you do the valuation in local currency and nominal terms? Thanks for your answers!

historical average