Can I change an outside light to an outside electric plug socket?
It's not hard to change an outside light fixture to a receptacle as long as you realize it's limitations. First your receptacle will be on a switch leg not a permanent power conductor unless you bypass the switch. You can bypass the switch but if there are any other lights on the switch that you wish to control you will need to get a source conductor to the receptacle.

To bypass a switch you simply have to remove the two conductors from the switch and tie them together with a wire nut. You can then blank off the switch or just leave it in place, but the splice must be left accessible. 

Also 2017 NEC pretty much requires all outdoor receptacles to be GFCI protected. The easiest way to do this is to install a GFCI receptacle and add a wet location cover.

Good luck.