In Arc GIS layout we can edit the contents in the legend of a layout by using the convert to graphics option and then ungrouping it. In the same way, is there any option is available in QGIS
You don't have to convert the legend to edit the text content, if you want to manually override the text you can do it from 'Item Properties'>'Legend Items', you just have to uncheck the 'Automatically Update' option, then edit the labels there.
And if you really do want to do something that isn't possible from there, you can export your layout as svg, and then edit the text and other graphic properties in the layout from your favorite vector image editor.