First you'll need to inspect the tension. Because chain rings and crank fingers are never perfectly round, the inspection and adjustment of the timing chain should be performed at the location of maximum tension or 'high spot'. Rotate the cranks until the chain is at its tightest. As you rotate, press down in the center of the top run of the timing chain and note the amount of vertical deflection allowed. Never adjust the chain tighter than this.
Some slack is necessary to accommodate frame flex.
Once it is determined that an adjustment is required, you will need to loosen the eccentric at the front bottom bracket. The clamps holding the eccentric in the bottom bracket shell will be either set screws in the shell, pinch bolts on a split shell, or internal expander bolts. In any case, one or two Allen-head screws are used. Loosen the eccentric with an Allen tool. Once loose, the eccentric can be rotated with a standard bottom bracket pin spanner.