You can access our previous releases here:
If your goal is to upgrade to LabKey 18.1, you do not necessarily need to go to 15.3 from 15.2 to then upgrade to 18.1 though.
You can use any of the previous releases prior to 18.1 to upgrade to, then upgrade to 18.1 right after.
So you can upgrade your current 15.2 instance to 16.2, 16.3, 17.1, 17.2, or 17.3. After upgrading to one of those, then you can upgrade to 18.1 successfully.
Just make sure that your current versions of Tomcat, Java, and Postgres (or MS SQL) are also up-to-date with the versions you're moving to.
I would recommend looking at these two Supported Technologies pages:
Safest scenario would be if you were running already on the latest version of Tomcat 7.0.X, Java 8.X, and at least PostgreSQL 9.3 or higher (or MS SQL 2012 or higher). This should allow you to avoid having to upgrade those components while upgrading the LabKey binaries.