how do i change photoshop CS5 to be compatible with big sur

See the following from https://helpx.adobe.com/x-productkb/global/fix-macos-64-bit-compatibility-errors.html

Most older Adobe apps were created using 32-bit architecture and do not work in Catalina. We have no plans to update apps that we no longer support or develop. The following table lists some common 32-bit Adobe apps that are not compatible with Catalina. (It is not a comprehensive list of all older 32-bit Adobe apps.)

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Creative Suite apps. No updates planned. We recommend that you upgrade to Creative Cloud.

It is not possible because CS5 - CS6 are 32-bit applications that were abandoned by Adobe years ago, and Big Sur only supports 64-bit only applications. You can choose to subscribe to Adobe for Photoshop 2021, purchase PhotoShop Elements 2021, or kick Adobe to the curb and acquire the $24.99 Affinity Designer/Photos (non-subscription). Many are doing just that. The Affinity products can be trialed from their website, and are also available in the Mac App Store.

Don't let the low price suggest that these are not professional featured applications ever bit as good as what Adobe rolls out for ransom.

You can't.