On 11 Dec, 17:26, wrote:

How do I sharpen it?

Many ways: various stones, or Scary Sharp (wet & dry stuck to glass,
web search for it) is probably your best bet.

I have got an angle grinder,

Don't use any sort of powered metalworking grinder to sharpen a chisel
- you'll overheat it and destroy the temper. If you were in a position
to do this, you'd have other and better options available to you.
Powered chisel grinders are too expensive, so go with the Scary Sharp.