macOS Catalina does not support 32-bit apps . Tools like Go64 can help you identify which applications will stop working after the update. macOS Catalina - Technical Specifications - Apple Support
Once the installer downloads, it will launch automatically. Follow the prompts. Note that your Mac will restart several times. If you are upgrading from High Sierra on a spinning Hard Drive (HDD) to an SSD file system (APFS), the installation process may take significantly longer than usual. how to update macos high sierra 10136 to 1015
Stuck at version 10.13.6, need 10.15. Not… - Apple Community 12 Feb 2021 — macOS Catalina does not support 32-bit apps
The download began—a hefty 8 GB file. Elias watched the progress bar under the Launchpad icon. It was slow, agonizingly so. He went to make coffee, then came back. It was at 50%. Follow the prompts