First, you have to insure the image you desire is not physically bigger than your flash. IOS images of newer versions and/or additional features tend to be larger. Sometimes, you'll need to drop down a feature set or version to find an image that will fit.
Second, insure your flash doesn't have any files on it that you no longer need. Often the IOS image you intend to replace will need to be removed to provide sufficient space for the new image.
Third, there's a delete command that can be used against individual files on flash but depending on the actual filesystem on the flash, the space may NOT be recovered just by deleting.
Fourth, to completely empty the flash, the erase command usually works or the newer format command. If supported, I would recommend the format command.
PS:
Not sure about the 1700s, but many later routers support one or more external flash modules. (Some even now support USB.) I.e. you might need to buy a bigger flash module. (Some internal flashes can also be upgraded; more involved.)
It's also possible to load and boot an IOS from an external server. Don't recommend this beyond the LAB.
