Not sure where to get the old version, but try these steps first:
Log out of Skype for Business/Lync.
Delete any sign-in info (onlyin Skype For Business 2013/2016)
Exit Skype for Business/Lync.
Navigate toUsers > username > AppData > Local > Microsoft > Office > x.0 (x.0 corresponds to the Skype for Business/Lync version: 16.0 = Skype for Business 2016, 15.0 = Skype for Business 2013, 14.0 = Lync 2010) > Lync
Delete sip_profileName folder.
Delete all files in Tracing folder. Do not delete the folder itself.
Clear DNS cache: in Command Prompt run ipconfig/flushdns command.