A pleasant Saturday afternoon ride interrupted by a mechanical problem ,I was going to shift down and as the shifter clicks nothing happens. The gears are stuck or can only move one or two steps, chances are it is a frayed cable within the shifter. I had this same problem on another bike so knew the signs. I was able to manually change the gears to find a mid range sprocket to get home.
Once home and with a replacement Shimano inner derailleur cable to hand I set about the repair. First release the cable with 5mm allen key from the rear derailleur and roll forward the rubber hood on the shifter to show route of the inner cable .
The hardest job is to make sure you have all the bits of frayed cable and the outer cables are clear to receive the replacement.
The new cable is fitted through the bottom of the shifter and routed through to the rear derailleur, making sure any adjusters are slackened to allow for maximum adjustment later.
Once the cable is in place it can be pulled taut, the retaining clamp nipped up and the gears indexed. On completion trim the cable and add an end cap to prevent the strands unravelling.