You can splice an outdoor cable in several ways. The easiest way to splice any cable is to cut it, then connect the two ends. You can use connectors and electrical tape, or you can buy a splice kit to secure the connection. Using connectors is the safer and more secure method and, done properly, gives a protected splice. As it is an outdoor cable, you will need to exclude moisture, so use a good connector with a silicon sealer. The splice should then be secured in an approved junction box.
Ensure that the electrical supply to the cable is turned off. Always be aware of health and safety.
Cut the cable and strip back the wires about an inch from both ends. Twist the wires together on each end in a clockwise direction.
Join the twisted ends using twist-on connectors; then secure the juncture with electrical tape.
Test the connection by turning the electrical supply back on. If all is well, place the splice in a junction box, and secure the box to a wall or bury it.
You could splice the cable using a splice kit that has shrink tubing. This tubing gives a neater finish and protects from water intrusion. If you use shrink tubing you will need a heat gun, plumber's torch or hair dryer at full blast to shrink the rubber tubing around the splice.