Oil pressure sending unit

A friend of mine has to change the sending unit for the oil pressure gauge on his 99 GMC with the 5.3 and apparantly its on the back of the block next to the firewall.Whats the easiest way to get at this to change it?Ive got basically the same truck only a Chev and if this happens to me I'd certainly like to know what I'm up against.Thanks

Unplug the connector, and unscrew from the block. If it is the small oil pressure sender then they make a special socket, I've never had to use one.