Once you create a ETABS file, it typically writes the input file as a .e2k or .$et file.
You should be able to open this in notepad or wordpad. Look for the following lines
$ FRAME SECTIONS
FRAMESECTION "W4X13" MATERIAL "STEEL" SHAPE "W4X13" FILE "D:\Program Files\Computers and Structures\ETABS 9\Sections8.pro"
FRAMESECTION "W5X16" MATERIAL "STEEL" SHAPE "W5X16"
You can define any materials you want in the model and it will be stored as a material definition. For example, in UcFSE's case, this could have been "A36", "A992" etc.
All you need to then do is to know which frame section gets assigned which property and then you can change it in the input.
As an example, if I wanted the W5x16 to be A36 steel, then all i would need to do is to change the material from "STEEL" to "A36"
It would then look like this
FRAMESECTION "W5X16" MATERIAL "A36" SHAPE "W5X16"
Once this is done, save the text file and import into ETABS.
File --> import --> e2k file.
The imported file will have the right assignment you need.
It is possible to change loads and other parameters, but this is rather tedious because each area element will have an ID and it is very convoluted to do it that way.