you can buy the tools/materials needed at most jewellery supply shops or over internet - look under Jewellery findings.
Yuo will need at least the following:
Torch/gas heat source.
(blowpipe-optional, skills needed.)
Gold solder (correct carat and type).
tools to hold item, such as tweezers.
Flux of correct type.
soldering board such as charcoal block.
``WARNING`` TO DO THIS REQUIRES HIGH DEGREE OF SKILL/ABILITY AND EXPERIANCE.
VERY HIGH PROBABILITY OF YOU MELTING THE LINKS OR INDEED THE ENTIRE BRACELET/NECKLACE
also high chance of getting severely burnt if you do not know what you are doing.
BEST TO TAKE ITEM TO QUALIFIED JEWELLERY REPAIRER/RESTORER.