For 2020 tax returns the official FTA exchange rates are:

USD1   =  CHF 0.93808189CHF 0.883944
EUR1   =  CHF 1.07045366CHF 1.081550
GBP1   =  CHF 1.20387835CHF 1.208300

Official foreign exchange rates as of 31 December 2020 are available here. In this list are also included rates to apply for the main cryptocurrencies. These need to be reported in the tax declaration.

BTC 1 = CHF 25476.030647
ETH 1 = CHF 650.931897

Average yearly exchange rates (for determining the income) are available here.