Yes, you may if you give credit but don't claim that they endorse your changes.
However I'm somewhat baffled... a license can IMHO not be more concise and clear and it's not even bad legalese... It's short enough to quote it in its entirety here (emphasis mine):
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from
this software without specific prior written permission.
As to changes:
"Redistributions of source code must retain the above copyright notice". So yes, you may amend copyright notices in the sources with your own, but you MUST keep the original one