There are a few ways to change volume, You can highlight a section you wish to change and use the 'Process> Volume' command.
Alternately, for custom fade slopes, you can use 'Process> Fade> Graphic' (use the Reset envelope and choose 200% which will be unity gain; double-click to add points and drag the points and/or envelope line up or down.
Using the "Event" mode is another way: 'Split' (S key) the timeline into sections, then use 'Gain' (drag the top of the event downward to lower) and/or add cross fades.
If you have Sound Forge Pro, the easiest (and best IMO), is adding volume automation envelope to a plug-in chain which can be adjusted over the entire timeline's duration. To enable the timeline volume automation and chainer, use the "V" keyboard shortcut and a envelope line will be created. (add points and drag like the aforementioned graphic fade process). To render the volume automation to the file, use Save or Save as. Screenshot of volume envelope. The envelope's line color may be different on your set-up. There are a few other work-a-rounds using a plug-in's input and/or output as well. Most plug-ins have at the least an output gain parameter.
btw, welcome to the Magix Sound Forge users community @Petals
In addition, when using envelopes to adjust volume of segments, if you right-click in-between the transition points, there are more options for the fade/transition types (linear, fast, slow, smooth, sharp, hold)