You won’t be able able to replace the top of the beam but you can cover it with something like this…

https://soundskins.com/product-category/beam/

Ok, thank you.. But the sound quality is OK, is the same with this cover?? 

Don’t have one myself but apparently the fabric is transparent to audio, a bit like cloth used on traditional speakers.

FWIW: I have no experience with the skins.

In theory the skin will absorb some of the high frequency energy. Even regular speakers sound slightly different with and without grills, but the significance of this difference varies from person to person. I expect that Trueplay can adjust most of this difference.

A consideration is that the skin will hide the exact location of the touch controls and may diminish their effectiveness. Also, voice control might become an issue.

In my opinion the Beam has the worse audio quality of all the Sonos speakers,  a fabric cover may actually improve it