I joined some fields to my tables and now I need a tool or something to change that field's name.
How can I do this in ModelBuilder?
I didn't find any tool to do that.
Feature Class to Feature Class would be your answer. Particularly Field Mapping part is what you are after. If you look at the example below, names of the three fields on the left table are altered to new ones with this tool.
One last note, if you have many fields to rename, this approach could be advantageous over the answer in terms of processing speed (it only takes as long as copy feature class operation), even though this approach may look complicated at first to some. In any case it involves manual editing/entering of each field in model builder either one by one or in bulk...
Another solution that is in concert with Aaron's solution,
arcpy.AlterField_management(r'C:\Data\Garbo.gdb\Khyber', 'oldfieldname', 'newfieldname', 'ALIAS') is the easiest way to achieve this.
There is the equivalent geoprocessing tool named "Alter Field" to be used in Model Builder.
This tool is available starting with 10.2.1
Before running the model make sure to close it in other application (e.g. ArcCatalog)