go get will install the package in the first directory listed at GOPATH (an environment variable which might contain a colon separated list of directories). You can use go get -u to update existing packages.
You can also use go get -u all to update all packages in your GOPATH
For larger projects, it might be reasonable to create different GOPATHs for each project, so that updating a library in project A wont cause issues in project B.
Type go help gopath to find out more about the GOPATH environment variable.