update the how-to-update-guide

[klin333]

Hi,

Thank you very much for the hard work on the latest release. So that others may self service or help out in the future, could you please document how to do the update? This script appears outdated. I tried the link with latest release number and the link leads me to 403 forbidden error.

blp/update

Lines 29 to 32 in ce09665

	## WINDOWS
	version=3.8.18.1
	cd $(mktemp -d)
	wget https://bloomberg.bintray.com/BLPAPI-Stable-Generic/blpapi_cpp_${version}-windows.zip

[eddelbuettel]

That script might not have been used. See its age:

PS I believe I accessed publically accessible Bloomberg URLs, I would not have had anything special.

PPS Starting here, likely: https://www.bloomberg.com/professional/support/api-library/