Excel tutorial: calculating covariance and correlation of stock returns
Discusses how to download two companies' stock returns from Yahoo Finance, and calculate (a) the variance and standard deviation of each stock, and (b) the covariance and correlation of the returns of both stocks.
Some good books on Excel and Finance:
Financial Modeling - by Benninga:
http://amzn.to/2tByGQ2
Principles of Finance with Excel - by Benninga:
http://amzn.to/2uaCyo6