If using Arduino IDE 2.x.x, use Boards Manager and click the drop-down list to switch between any of the versions which support the newer IDE.
If using old Arduino IDE 1.8.x, go to the download page, scroll down to the section for Arduino 1.8, and right click the download link. If using Mac with single button mouse, hold control key while clicking. Select the option to copy link. Then in your browser address, paste but do not press enter. Look for the 3 digit version number. Edit to "157". Then press Enter to download the old installer.