First of all it has to be a sheetmetal part, if it is a solid part then convert it. You need to add at least 1 sheetmetal specific feature in the conversion process. A rip is a good one as you will need it anyway. Rip down one of the bends.
Then set a reference geometry, this is the part thet remains in the same plane when you unbend the model. If you do an unbend it will ask you to set the reference or you can do it separately. I usually set it in the early stages of the design.
Then just do an unbend. The unbend is usually done once at the end of the design. If you do it early for some reason you can then delete it to save regen time unless you are using a reference from it.