1. To ensure someone has the most current and accurate information (about something). Please keep me up to date regarding the progress of the project. Now that I'm in college, my parents want me to keep them up to date about everything going on in my life.

2. To ensure something is the most current version available. (Often used in reference to technology.) Please make sure to keep your phone's operating system up to date. I don't know where you find the time or money to keep your fashion up to date like that.

3. (used without an object) To have or be informed about the most current and accurate information (about something). I try to make a point of keeping up to date with news from around the world.