I have High Sierra on a 2011 MacBookPro, but now need to downgrade to an older OS inorder to be compatible with olde softwarebe compati

JROJr wrote:

maybe to the version prior to High Sierra or two back.

Revert your Mac to a previous macOS version - Apple Support

JROJr wrote:

I have High Sierra on a 2011 MacBookPro, but now need to downgrade to an older OS in order to be compatible with older software. Is this possible, and how to install it?

Older than High Sierra— how far back do you want to go?

You are better off updating the outdated software.. forward not backward.

As it is, Support for your High Sierra ends Nov. 30 2020