It is very easy to upgrade the whole Debian system, but you can also upgrade only selected packages and skip the rest.

List available updates by simulating upgrade process.

$ sudo apt-get upgrade -s
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be upgraded:
  gnupg gpgv libgcrypt11 libssl-dev libssl-doc libssl1.0.0 linux-headers-3.2.0-4-amd64 linux-headers-3.2.0-4-common linux-image-3.2.0-4-amd64 linux-libc-dev openssl
11 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Inst libgcrypt11 [1.5.0-5+deb7u4] (1.5.0-5+deb7u5 Debian-Security:7.0/oldstable [amd64])
Inst libssl-doc [1.0.1e-2+deb7u21] (1.0.1t-1+deb7u1 Debian-Security:7.0/oldstable [all])
Inst libssl-dev [1.0.1e-2+deb7u21] (1.0.1t-1+deb7u1 Debian-Security:7.0/oldstable [amd64]) []
Inst libssl1.0.0 [1.0.1e-2+deb7u21] (1.0.1t-1+deb7u1 Debian-Security:7.0/oldstable [amd64])
Inst linux-image-3.2.0-4-amd64 [3.2.81-1] (3.2.81-2 Debian-Security:7.0/oldstable [amd64])
Inst gpgv [1.4.12-7+deb7u7] (1.4.12-7+deb7u8 Debian-Security:7.0/oldstable [amd64])
Conf gpgv (1.4.12-7+deb7u8 Debian-Security:7.0/oldstable [amd64])
Inst gnupg [1.4.12-7+deb7u7] (1.4.12-7+deb7u8 Debian-Security:7.0/oldstable [amd64])
Conf gnupg (1.4.12-7+deb7u8 Debian-Security:7.0/oldstable [amd64])
Inst linux-headers-3.2.0-4-amd64 [3.2.81-1] (3.2.81-2 Debian-Security:7.0/oldstable [amd64]) []
Inst linux-headers-3.2.0-4-common [3.2.81-1] (3.2.81-2 Debian-Security:7.0/oldstable [amd64])
Inst linux-libc-dev [3.2.81-1] (3.2.81-2 Debian-Security:7.0/oldstable [amd64])
Inst openssl [1.0.1e-2+deb7u21] (1.0.1t-1+deb7u1 Debian-Security:7.0/oldstable [amd64])
Conf libgcrypt11 (1.5.0-5+deb7u5 Debian-Security:7.0/oldstable [amd64])
Conf libssl-doc (1.0.1t-1+deb7u1 Debian-Security:7.0/oldstable [all])
Conf libssl1.0.0 (1.0.1t-1+deb7u1 Debian-Security:7.0/oldstable [amd64])
Conf libssl-dev (1.0.1t-1+deb7u1 Debian-Security:7.0/oldstable [amd64])
Conf linux-image-3.2.0-4-amd64 (3.2.81-2 Debian-Security:7.0/oldstable [amd64])
Conf linux-headers-3.2.0-4-common (3.2.81-2 Debian-Security:7.0/oldstable [amd64])
Conf linux-headers-3.2.0-4-amd64 (3.2.81-2 Debian-Security:7.0/oldstable [amd64])
Conf linux-libc-dev (3.2.81-2 Debian-Security:7.0/oldstable [amd64])
Conf openssl (1.0.1t-1+deb7u1 Debian-Security:7.0/oldstable [amd64])

Install updates, but skip these related to Linux kernel.

$ apt-get install --only-upgrade libgcrypt11 libssl-dev libssl-doc libssl1.0.0 linux-libc-dev openssl
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
The following packages will be upgraded:
  libgcrypt11 libssl-dev libssl-doc libssl1.0.0 linux-libc-dev openssl
6 upgraded, 0 newly installed, 0 to remove and 5 not upgraded.
Need to get 5857 kB/6158 kB of archives.
After this operation, 265 kB of additional disk space will be used.
Get:1 wheezy/updates/main libssl-doc all 1.0.1t-1+deb7u1 [1224 kB]
Get:2 wheezy/updates/main libssl-dev amd64 1.0.1t-1+deb7u1 [1791 kB]
Get:3 wheezy/updates/main libssl1.0.0 amd64 1.0.1t-1+deb7u1 [1283 kB]
Get:4 wheezy/updates/main linux-libc-dev amd64 3.2.81-2 [858 kB]
Get:5 wheezy/updates/main openssl amd64 1.0.1t-1+deb7u1 [700 kB]
Fetched 4735 kB in 2s (1990 kB/s)
Reading changelogs... Done
Preconfiguring packages ...
(Reading database ... 58380 files and directories currently installed.)
Preparing to replace libgcrypt11:amd64 1.5.0-5+deb7u4 (using .../libgcrypt11_1.5.0-5+deb7u5_amd64.deb) ...
Unpacking replacement libgcrypt11:amd64 ...
Preparing to replace libssl-doc 1.0.1e-2+deb7u21 (using .../libssl-doc_1.0.1t-1+deb7u1_all.deb) ...
Unpacking replacement libssl-doc ...
Preparing to replace libssl-dev 1.0.1e-2+deb7u21 (using .../libssl-dev_1.0.1t-1+deb7u1_amd64.deb) ...
Unpacking replacement libssl-dev ...
Preparing to replace libssl1.0.0:amd64 1.0.1e-2+deb7u21 (using .../libssl1.0.0_1.0.1t-1+deb7u1_amd64.deb) ...
Unpacking replacement libssl1.0.0:amd64 ...
Preparing to replace linux-libc-dev:amd64 3.2.81-1 (using .../linux-libc-dev_3.2.81-2_amd64.deb) ...
Unpacking replacement linux-libc-dev:amd64 ...
Preparing to replace openssl 1.0.1e-2+deb7u21 (using .../openssl_1.0.1t-1+deb7u1_amd64.deb) ...
Unpacking replacement openssl ...
Processing triggers for man-db ...
Setting up libgcrypt11:amd64 (1.5.0-5+deb7u5) ...
Setting up libssl-doc (1.0.1t-1+deb7u1) ...
Setting up libssl1.0.0:amd64 (1.0.1t-1+deb7u1) ...
Setting up libssl-dev (1.0.1t-1+deb7u1) ...
Setting up linux-libc-dev:amd64 (3.2.81-2) ...
Setting up openssl (1.0.1t-1+deb7u1) ...

This command will upgrade only installed packages, it will ignore non installed ones.