update cupons c 
INNER JOIN (select m.nfce AS 'nfce',
				sum(m.desconto)+sum(m.desconto_item) as 'desconto' 
				from movdiario m 
				where m.emissao between '20200801' and '20200909' 
				and m.SITUACAO is null 
				and m.nfce <> 0 
				group by m.nfce) t ON c.NFCE = t.nfce
SET c.VL_DESCONTO = t.desconto
where c.cstat not in (100,102,135,150) 
and c.emissao between '20200801' and '20200909' 
and c.tipo = 'RV - CUPOM FISCAL'
and c.nfce <> 0