OS X Yosemite’s tenacious Wi-Fi connectivity issues have been well publicized: Apple issued a fix with the 10.10.1 update, but apparently, issues persisted for many. But Apple’s giving it another go with the newly released 10.10.2 update.
In addition to the Wi-Fi fixes, this update also corrects bugs and improves performance in Safari, Mail, VoiceOver, and iCloud, among other areas of the OS, according to the release notes.
You can get the 10.10.2 update through the Mac App Store’s Update tab. Good luck and happy updating.