1. First, assign a new UID to user using the usermod command.
2. Second, assign a new GID to group using the groupmod command.
3. Finally, use the chown and chgrp commands to change old UID and GID respectively. You can automate this with the help of find command.

It cannot be stressed enough how important it is to make a backup of your system before you do this. Make a backup. Let us say:
Our sample user name: foo

1. Foo’s old UID: 1005
2. Foo’s new UID: 2005
3. Our sample group name: foo
4. Foo’s old GID: 2000
5. Foo’s new GID: 3000

Commands

To assign a new UID to user called foo, enter:
# usermod -u 2005 foo
To assign a new GID to group called foo, enter:
# groupmod -g 3000 foo
Please note that all files which are located in the user’s home directory will have the file UID changed automatically as soon as you type above two command. However, files outside user’s home directory need to be changed manually. To manually change files with old GID and UID respectively, enter:
# find / -group 2000 -exec chgrp -h foo {} \;
# find / -user 1005 -exec chown -h foo {} \;

The -exec command executes chgrp or chmod command on each file. The -h option passed to the chgrp/chmod command affect each symbolic link instead of any referenced file. Use the following command to verify the same:
# ls -l /home/foo/
# id -u foo
# id -g foo
# grep foo /etc/passwd
# grep foo /etc/group