First always depends on professional advice. First you have to apply for a legal heir certificate from revenue authorities and submit an original death certificate. They will make inquiries and provide you a list of legal heirs of your GM. If your father is the only legal heir of your mother, you can apply for such a mutation process. The process is simple. You have to apply in prescribed format and remit fees. Even in case of a valid will, you can contact an advocate and follow the laid down procedure for mutating the property in your father's name.
If there is more than one legal heir to your GM property, you have to enter into a family settlement written deed, get the shape of award from Legal services authority, and get the deed indexed through them.
The costs are most reasonable and may not exceed Rs.2,000/- in all. Some municipalities may impose penalties for late mutation.