I'd suggest probably getting it to someone with some experience if you're not sure. Its not a hard job by any means, but you're already dealing with an out of the norm issue. If the clicker is corroded into the damper rod theres a good chance there'll be corrosion in other parts of the fork cap/upper leg.
But, if you want to have a go there is loads of info and videos on youtube. Most Open Cart forks are very similar.
Loosen the upper triples, use appropriate sized tool on the cap (varies depending on fork and cap design) and loosen the cap. Remove fork from bike and completely undo the fork cap. The upper will slide down to expose the cap and spring. You then need to pull the spring down, use a spanner on the lower part, socket/tool on the cap and back the cap off from the damper rod.
With the Sachs, you'll likely find that your broken/sheered rebound adjuster will remain in the damper rod while the cap comes off.