You will need to do incremental upgrades.
Detailed Upgrade Instructions for 1.5.x, and versions 2.0 - 2.6.5, to 3.4.1
The following are the instructions that explain the above steps in more detail. Please read all of the instructions before starting.
If you plan on upgrading across more than two major releases, please consider upgrading incrementally to avoid potential conflicts. For example, if you plan on upgrading from 2.5 to 3.4.1, upgrade to 2.7 first, followed by 2.9, and finally 3.4.1. Essential, it's okay to skip one release, but never skip TWO when upgrading.