Scrape and hard brush off as much as you can and then use a proprietary moss killer.
If its very bad you'll need to let it die off for a few weeks, brush again and apply again.
If its really awful power wash it, powerwashing isn't great for tarmac but its the only way to get it back to reasonably clear sometimes so go gently if you do powerhose it.
Good ones include brittons mmc pro or the retail equivalent called patio magic is ok.
Mossgo is also quite good and good old fashioned armillatox is good if you can bare the smell.
The secret to controlling moss is to apply the chemical regularly as the surface begins to green a little. Once you actually get fluffy growth it becomes much more difficult.
Don't use iron, sulphate of iron, ferrous sulphate (all the same thing) it will turn it orange but some idiots still recommend it because its used on lawns and is cheap.
Whatever you use don't skimp on the application rate, it needs to be thourougly wetted