You can use a change node and JSONata expression set to $base64decode(payload)
Or there is a Base64 contrib node.
example
[{"id":"3fd3fa3f.9e49b6","type":"inject","z":"b779de97.b1b46","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"PAAAelFhAAAABwEBMgAAAAAAAQBMBOIEAAAAAAAAPADWK+sVgACCQgAADQCAAIJCHJNHYQAAAAAAAAAAAQBdAQ==","payloadType":"str","x":120,"y":4560,"wires":[["cd62cb00.fe0e28","f902ba.890ccd48"]]},{"id":"cd62cb00.fe0e28","type":"change","z":"b779de97.b1b46","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$base64decode($$.payload)\t","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":350,"y":4560,"wires":[["37ca434a.148d5c"]]},{"id":"f902ba.890ccd48","type":"base64","z":"b779de97.b1b46","name":"","action":"b64","property":"payload","x":350,"y":4520,"wires":[["37ca434a.148d5c"]]},{"id":"37ca434a.148d5c","type":"debug","z":"b779de97.b1b46","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":580,"y":4560,"wires":[]}]