change thermal paste

hi, about a month ago I bought my first raven coin mining rig (Radeon RX 570). it works fine but the tempeture is quite high for the settings its running. I asked my friends who know a bit more about computers and they said it might be time to replace the thermal paste, the mining rig is quite old(I bought is second handed). how do I know when to replace thermal paste en what thermal paste is recommended?