Hammer and chisel or use a breaker

Your best course of action would be to first establish what materials have been used to block the opening and what type of fireplace was there when it was being used.
You should check for any gas or electrical outlets in the close vicinity and if necessary get qualified tradesmen in to remove anything live.
Once the area is safe you should carefully drill a hole in the centre of the old opening and check what is behind in the old fireplace. Ensure that the wall above the old opening is supported and then you should be able to remove the section quite easily.
If in doubt contact your Local Authority Building Control Department for advice.

I would measure the back of the fireplace insert, taking note of the size of opening, the insert requires to fit into the chimney breast.
然后我将钻定位孔的the area the insert will fit into, I usually mark the wall, then drill the pilot hole, then, Chip the plaster around the hole away to revile the bricks, use the drill to remove the cement around the brick, to form a brick sized hole in the chimney breast.
Using a torch try to look inside the chimney breast, if it’s hollow, most of them are.
Measure the distance to the inside of the chimney breast, either side, inside the opening.
This will allow you to confirm the size of the opening, that the chimney breast will open up to.
If the opening is too big, I would take out the builders opening, rebuild the brickwork to the size required, fit the insert, then replaster.
Don’t forget to fit a nice granite hearth!!!

Thanks

If it's all brick then you will most likely need a lintel to support the chimney above. With the way bricks lay you will need to cut some half bricks to fill in the sides. The rest of the opening is simple to remove. If it is just plastered over then you may find the original chimney behind. If you doing it yourself tread carefully or better use a certified installer/builder.