You must not shave your beard.

However, it is permitted to dye the hair with henna or any other substance that will give it a reddish or brownish colour, because itl is reported that the Prophet sallAllahu alayhi wa salaam, used to dye
his hair with yellow. Muslim reported that Abu Bakr dyed his hair with henna and katam and that ‘Umar used to dye his hair with henna. The Prophet sallAllahu alayhi wa salaam also said, “The best things with
which you can change this grey hair are henna and katam.”