I suggest you ask the Enigmail folk.

A quirk google search took me here https://www.enigmail.net/index.php/en/faq?view=category&id=13

where they say

The latest version of Enigmail, v1.9, does not support GnuPG 1.4 anymore. This article provides tips and help for migrating from GnuPG 1.4 to GnuPG 2.0. Enigmail supported GnuPG 1.4 since 2004, 2.0 since 2007 and 2.1 since 2015. Enigmail 1.8 supports them all, but GnuPG 2.0 is recommended since Enigmail 1.6. Enigmail 1.9, the next major version of Enigmail will require GnuPG 2.x. That is why GnuPG 1.4 is now deprecated.

Installation

Windows

• We recommend you install Gpg4win-Vanilla from gpg4win.org. This will install all components required by Enigmail. Expert users may install the full version instead. Its additional parts are not used by Enigmail.

Linux and other Unix Systems

• We recommend you install GnuPG 2.0 via the regular package management system of your distribution (e.g. apt, yum, yast). On many distributions the package is called "gnupg2" or "gpg2".

Mac OS X

• We recommend you download GpgTools and install only the core parts. Expert users may install all components. The additional components are not used by Enigmail.

General Recommendations

There is no need to uninstall GnuPG 1.4.x; in most cases Enigmail will find GnuPG 2.0 automatically after the installation. Simply restart Thunderbird after you installed GnuPG 2.0.x. If Thundebird would still report that it found GnuPG 1.4.x, then uninstall the GnuPG 1.4.x package and restart your PC afterwards.

Make sure to keep Thunderbird, Enigmail and GnuPG up to date. Only the latest release version of GnuPG / Gpg4Win / GpgTools is supported by Enigmail for security reasons. This applies to GnuPG stable (2.0.x) and modern (2.1.x). What happens to my keys after the upgrade?

GnuPG 2.0 uses the same keys as GnuPG 1.4; there is no need to change anything concerning your private and public keys. However, independently of whether or not you upgrade GnuPG, we recommend you make a backup of your keys from time to time. Now is the perfect moment to do this.

Thank you very much Matt,

Unfortunately that doesn't help much. I'm using Ubuntu Linux but I'm only a normal user, and I don't understand what "the regular package management system of your distribution" means.

I tried the support forum at Enigmail but I couldn't find any answers.

What I need is a step by step How to guide.

What I need is a step by step How to guide.

What you need is a basic understanding of "the regular package management system of your distribution", Ubuntu in your case. There's lots of documentation for Ubuntu. And I suppose they do have a support forum as well. Google is your friend.

I got help to solve my problem at "Ask Ubuntu":

http://askubuntu.com/questions/748057/how-do-i-install-gnupg-2-0