Using AWK, If, and SED

awk '{ if (NR -ge $num && NR -le $num_max && $1 == $UM);

then sed 's/'$UM'/'$UM_lower'/g'dipping_CR_UM_change.txt > new.txt}'

I get the following error

awk: syntax error at source line 1 context is { if (NR -ge $num && NR -le $num_max && $1 == $UM); then sed >>> s/$UM/$UM_lower/gdipping_CR_UM_change. <<< txt > new.txt}

awk: illegal statement at source line 1