XSLT: Change certain attribute values

I am a XSLT newbie and have a simple task:

Suppose I have the following XML:

<Element1>
    <Element2 attr1="1"/>
</Element1>
<Element1 attr1="2"/>
<Element1>
    <Element2 attr1="2"/>
</Element1>

I want to transform the XML to the same XML with one change: All attributes named "attr1" no matter where they are have to be transformed so that for example "1" will be "A" and "2" will be "X", i. e. to

<Element1>
    <Element2 attr1="A"/>
</Element1>
<Element1 attr1="X"/>
<Element1>
    <Element2 attr1="X"/>
</Element1>

How can I achieve this? Thanks in advance!