Use this guide to completely replace your mini's hard drive.
Note that some of the connections in this Mac are more fragile to disconnect and more difficult to reconnect than in other Macs. This guide is thorough in explaining how to remove each part that's in the way of the hard drive, but you may want to skip some steps and move the fan and the antenna plate out of the way without disconnecting them, to avoid undue hassle and large repair bills if you break a connector. Also, you may be able to replace the hard drive without removing the motherboard. Read the comments on the steps below before you proceed - you may be glad you did.