With or without reformatting? With reformatting is easy, FDisk will do it if you tell it to delete the partitions then create a new one from the total space.
Withour reformatting means you'd need to install something like Partition Magic and adjust the partition, it should work without issues but I'd definitely suggest backing up anything important - it is a good general rule to do so before attempting anything that might mess up your PC. It does however cost money so it depends how much you don't want to reformat if it is worthwhile.