How do i adjust the thrust of the engines in xml?

Keep in mind that throat is limited to 0.001, negative nozzles can be messy, high pressures lead to inefficient vaccum engines and @Kell may have a heart attack if he finds out they're not stock

Get an xml editor and insert the file. there are many different aspects you can change, but you cant out right change the thrust. you can change the size and scale down the engine or increase chamber pressure or throat radius, xml lets you change change the properties to negative or thousands of time the stock value