The SeaMonkey installer for Windows, Linux, and Mac OS X can be downloaded from the SeaMonkey Download & Releases page, which includes a link to the Release Notes. The release notes contain detailed installation instructions, system requirements, what's new, and a list of known issues.
Upgrading from Mozilla Suite to SeaMonkey 1.x
You can uninstall Mozilla Suite either before or after installing SeaMonkey 1.x. Any settings will still be in your profile folder so you will not lose any information. It is advisable to make a backup of your profile - just in case.
You may want to back up your Mozilla Suite plugins folder (inside the installation directory) so that any plugins stored there can be copied back to the new SeaMonkey installation directory plugins folder. (Go to "Help -> About Plug-ins" to see the locations of the plug-ins Mozilla is currently using.)
Extensions installed in the Mozilla Suite installation directory will have to be reinstalled. Extensions installed in your profile will still be there after installation. Uninstalling Mozilla will leave your profile and your user preferences intact and SeaMonkey 1.x will automatically use your Mozilla Suite profile.
Upgrading from SeaMonkey 1.x to SeaMonkey 2
SeaMonkey 2 is available from the SeaMonkey Download & Releases page.
If you want to have both SeaMonkey 1.x and SeaMonkey 2 installed, choose a different destination folder for SeaMonkey 2.  Installation instructions and additional information can be found here.
SeaMonkey 1.x and SeaMonkey 2 store user profiles in different profile folder locations and do not use each other's profiles. When you first start SeaMonkey 2 after installation, the "Import Wizard" will launch and will offer to migrate an existing Mozilla Suite/SeaMonkey 1.x profile. . For more information on profile migration in SeaMonkey 2, see Profile migration - SeaMonkey.