Debian 7 (Wheezy) is still only shipping with Subversion 1.6 (see Details of package subversion in wheezy).

First you need to add the WANdisco GPG key to the apt sources keyring:

$ wget -q http://opensource.wandisco.com/wandisco-debian.gpg -O- | apt-key add -

Then you need to add the appropriate apt source. You can either extend /etc/apt/sources.list or create a new file in /etc/apt/sources.list.d. We’ll go for the second option:

$ echo "deb http://staging.opensource.wandisco.com/debian wheezy svn18" > /etc/apt/sources.list.d/subversion.list

Now you can have apt-get download the package lists from the new repository and update the dependencies:

$ apt-get update

Now apt-cache will show you (at least) two versions:

$ apt-cache show subversion
Package: subversion
Version: 1.8.8-3+WANdisco
Architecture: amd64
Maintainer: Philip Herron <opensource@wandisco.com>
Installed-Size: 4423
Depends: libsvn1 (= 1.8.8-3+WANdisco), libapr1 (>= 1.3.2), libaprutil1 (>= 1.2.7+dfsg), libc6 (>= 2.3), libexpat1 (>= 2.0.1), libsasl2-2 (>= 2.1.24), zlib1g (>= 1:1.1.4)
Suggests: subversion-tools, db4.8-util, patch
Filename: dists/wheezy/svn18/binary-amd64/subversion_1.8.8-3+WANdisco_amd64.deb
Size: 1453080
MD5sum: 8b23345878430ab7c5d22aabd461bbd6
SHA1: bc8125dfcc3c7bee34b09eca6919d5e64a7513d4
SHA256: 861987aebdcd2fa9f03ba9f758572fb05f8b345341533c0d072b5ef573443999
Section: vcs
Priority: optional
Homepage: http://wandisco.com
Description: Advanced version control system

Package: subversion
Version: 1.6.17dfsg-4+deb7u4
Installed-Size: 4108
Maintainer: Peter Samuelson <peter@p12n.org>
Architecture: amd64
Depends: libsvn1 (= 1.6.17dfsg-4+deb7u4), libapr1 (>= 1.2.7), libc6 (>= 2.3), libsasl2-2 (>= 2.1.24)
Suggests: subversion-tools, db5.1-util, patch
Description: Advanced version control system
Homepage: http://subversion.apache.org/
Description-md5: 15da1bb51fb2e9ea5e25b3a489b864d9
Tag: devel::rcs, implemented-in::c++, implemented-in::python,
 interface::commandline, network::client, network::server,
 protocol::http, protocol::ssh, protocol::webdav, role::program,
 use::storing, use::synchronizing
Section: vcs
Priority: optional
Filename: pool/main/s/subversion/subversion_1.6.17dfsg-4+deb7u4_amd64.deb
Size: 1319636
MD5sum: 608ed74ad8856546afd0a38ad171d0a5
SHA1: 46b53dc63ae20c59c39e152d65d68397c951051f
SHA256: a4a7fd1aefc7f160aa4a270792e5b7812a3804623d36b34f41055ed7a4b083a0

You can now install the new version using:

$ apt-get install subversion

Now Subversion 1.8 will be installed, you can check it with:

$ svn --version
svn, Version 1.8.8 (r1568071)
...

If you have already done all this for Subversion 1.7 and only want to upgrade to versino 1.8, it’s even easier. You just have to find where the source is defined (you don’t need to import the key as you already have it):

$ grep -R svn17 /etc/apt/
/etc/apt/sources.list.d/WANdisco.list:deb http://staging.opensource.wandisco.com/debian wheezy svn17

Then replace svn17 by svn18 in this file:

$ sed -i 's/svn17/svn18/' /etc/apt/sources.list.d/WANdisco.list

Now you can have apt-get download the package lists from the new repository and upgrade subversion:

$ apt-get update && apt-get upgrade