If you want to make changes to your loan, then you will have to discuss it with your lender. If you have a local branch, it will be best to go in and chat to someone face to face about your requirements. Otherwise you may need to speak to someone in the customer services department over the telephone. It can be best for you to find a way to discuss it that you are comfortable with. You may find that it will take a series of discussions for you and your lender to be able to come up with a decision that suits you both.
It will not be something that can be organised quickly and easily. The lender will want to check the details of the existing agreement and then decide whether the proposed changes will be suitable for them. There is no guarantee that they will be able to modify a loan agreement. If they can make it pay for them, then they are more likely to agree and if you have a good credit rating. If you have had difficulty making repayments, then they could be reluctant to lend you more money, but they might be happy to work with you on finding a repayment schedule that you can more easily manage.
The lender wants to protect themselves in the first instance. If you are having trouble paying and want to reduce payments and extend the loan, they will probably agree as long as your proposal is reasonable. They would rather that you were able to pay back the loan than if they had to repossess the house and sell the property as this often means they do not get back the full value of the loan, depending on the amount of equity in the property. They are likely to only make the loan bigger if the house has got significant equity in it. With the economy being poor and house prices not rising that much, they will be cautious in doing this.
So the first step is to discuss your situation and needs to your lender and see whether they can help you. It is likely that you will have to work together to find something that will suit both of your needs. They will want to make sure they get back the money they have lent in the future and make a profit out of it and so they will need to look after their own interests as well as yours.