Shift into your highest gear - the smallest sprocket at the centre of the rear wheel - ensuring that the shifter is in the position that lets the cable be as loose as possible.
Move the shifter one click and turn the cranks. The chain should move up one sprocket. If it doesn't, find the barrel adjuster where the cable enters your rear derailleur and turn it anticlockwise with your fingers about a half turn at a time until it does.
Move the shifter another click and turn the cranks again. If the chain doesn't shift to the third smallest sprocket, turn the barrel adjuster about quarter of a turn at a time until it does.
Check that the shifts to the smaller sprockets are working fine. Keep adjusting - anticlockwise to tighten the cable, clockwise to loosen it - until the gears are back in sync. If you can't get the cable tight enough to move the chain up with one click of the lever, there might be another adjuster by the shift lever that works in a similar way, and/or one on the down tube cable stop. If not, loosen off the anchor bolt, pull the slack cable tight and start the process again.
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