Then you need to remove the faulty unit and replace it with a good one. Just follow the procedure described at p.19 on the [link|https://dl.dell.com/topicspdf/optiplex-7020-desktop_owners-manual_en-us.pdf|user manual|new_window=true], the size of the PSU you need to buy varies depending on the specific model you have, a common PS2 unit should fit into the tower, but you'd need an SFF unit (like [link|https://www.amazon.com/H255ES-00-Precision-D255AS-00-D255E001L-F255ES-00/dp/B082RTLXKM/ref=sr_1_4?crid=1CK9NM4ZBP00T&keywords=SFF+psu+dell+7020&qid=1671013706&sprefix=sff+ps+dell+7020%2Caps%2C274&sr=8-4|this|new_window=true] one) if you happen to have the smaller model. Here's a [link|http://www.evercase.co.uk/PowerSupplyMeasurement.htm|list|new_window=true] of the most common PSU form factors.
Let us know how the repair goes and don't hesitate to ask if you need more advice