Hello jean, welcome to the forums. I will leave your question here but would you mind saying hello to every one in the New Members forum and tell us a bit about yourself. You will get plenty of help with your query I am sure.
Doing a re-grout is not an easy task. The old grout needs to be removed carefully and this can be done either by hand or with the aid of machinery. You didn't say if it was walls or floor and how much area you need to re-do.
If it is a large area a machine like a Fein Multimaster with grout removal attachement can make like much easier but it is an expensive item to buy, around £160, the only other way of doing it is manually with a grout rake. This is very hard work and tedious, and also very messy. You also have the possibility of causing damage to your tiles. Consider your options carefully. It may be easier, quicker and cheaper to re-tile.
Grumpy
