If you mean converting e.g. "13.37" to "13.37 %", this is done by converting the float into a string.
This can be done by doing a change object. You need, however, a different attribute in your entity. For instance, if the float-attribute is called $Entity/FloatValue you could call your string-attribute $Entity/StringValue.
What you then do, is create a microflow in which you have an activity that changes the StringValue object. It should return the exact following value:
toString($Entity/FloatValue) + ' %'
That's all :)