Hii Sasi ,
Go to CMC and go to Respective folder where reports are saved under " Folder " option .
click on Folder " Tom " and click on " Manage user Security " and then under " user security " you will find the particular user " Tom " whom Modify rights need to be given then click on that user id and click on " Assign Security " option on Top right side and here you can Drag " Full Control " rights from Available access level to assigned access level .
And your user will get modification rights
Regards
Jeetan