How do I change my timezone to UTC/GMT?

Any idea how I change from IST to GMT?

To switch to UTC, simply execute sudo dpkg-reconfigure tzdata, scroll to the bottom of the Continents list and select Etc or None of the above; in the second list, select UTC. If you prefer GMT instead of UTC, it's just above UTC in that list. :)

sudo timedatectl set-timezone UTC

Will change your timezone to UTC system wide.

You can do timedatectl list-timezones to see all the available timezones.

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excerpt from @MattJohnson's answer on SO

Zones like Etc/GMT+6 are intentionally reversed for backwards compatibility with POSIX standards. See the comments in this file.

You should almost never need to use these zones. Instead you should be using a fully named time zone like America/New_York or Europe/London or whatever is appropriate for your location. Refer to the list here.

In the old Un*x style (SunOS, HPUX...), you can do:

ln -fs /usr/share/zoneinfo/UTC /etc/localtime

Check the contents of /usr/share/zoneinfo to get the timezone you want/need.

For instance, Irish Summer Time (IST) can be defined as

-rw-r--r-- 1 root root 3661 Mar 13 22:18 /usr/share/zoneinfo/posix/Eire

or

lrwxrwxrwx 1 root root 13 Mar 13 22:18 /usr/share/zoneinfo/Europe/Dublin -> ../posix/Eire

However, the most important is to use a proper clock reference and a ntp daemon (openntpd for instance), as timezone is only used for displaying/converting the time to strings, not to store it (whatever the timezone, difference to 01/01/1970 is everywhere the same on Earth).