My sofa is so dirty

You can get upholstery cleaner, often comes with a sort of applicator.

Have a look in the supermarket.

I have a couple of furniture charity stores near us British heart foundation and YMCA .
I have seen the staff is their stores doing a very good job of cleaning used furniture.

There must be others who could do a good job like that.
It can be done with really experienced staff.

I got dye on my sofa as few years back. Contacted the contents insurance company and they arranged to have it cleaned by a specialist under the claim

My sofa is generally grubby on the arms. I've tried a couple of supermarket bought upholstery cleaners, not very successfully.
There are a few professional cleaners of carpets and upholstery in my area. I will be booking one soon.

I'd call a professional, as ninathenana says people who clean carpets usually do upholstery as well. Not something I've ever done as my sofa's have always been leather since around 1980.

I have a professional carpet and upholstery cleaner in every so often to do mine . They charge £30 per chair and £90 a 3 seater sofa in our area.

Anther who would get the professionals in.

I do not think it worth it to tackle it yourself. I also would call in a professional. It is not everyday wear and tear, this is a specific liquid spilt on it.

Yes call in the professionals. We tried cleaning our ourselves with a little handheld cleaner. It ended up striped!

Replace it.

Supermarket bought upholstery cleaners are useless. They do not do what it says on the tin. Take the advice above to call in the professionals and meantime throw a throw over the sofa.

When I bought my sofa I was told it is not possible to wash covers or get them dry-cleaned as this removes the fire protection.