remove cable/housing in shifters

how do you remove those from the shifters (shimano 105) ? do you have to completely disassemble the shifter? because i don't see where i can remove it for replacing new ones...

 

Check out the Park Tools website, they have great instructions... Also the technical documents section on the Shimano website. There also many videos in YouTube.
You pull the brake lever to expose the cable anchors. Undo the cable anchors at the brakes and derailleurs, open (by applying pressure as if you were braking) the brake lever, then grab one of the cable ends you just unanchored...say the front brake cable, and push up on the cable a little bit. As you open the brake lever and push the cable up, you will see the cable end popping up inside the shifter. You'll need needle nose pliers to grab the cable end (it looks like a little barrel) and pull it. You'll have to remove the handlebar tape. The shifter cables are similar only they come out sideways and the cable ends are round. Make sure you downshift to the lowest gear in front (small chainring) and highest on back (smallest cog) BEFORE YOU remove the cable at the DR anchor bolts and screw in all the barrel adjusters (clockwise) to release all the tension.
If the DR limits were set correctly before you replaced the cables, all you have to do is reset the indexing. Also check the Park Tools site for this.