<time>20260664</time>
<tags>substancesummit ss</time>
<geo>asdsadsa</geo>
<time>20260664</time>
<tags>substancesummit ss</time>
<geo>asdsadsa</geo>

sed 's/time>n<geo>/tags>n<geo>/g' old.xml > new.xml

sed -e '/^<tags>/ s!</time>$!</tags>!'

sed '$!N;s/ime(>n<geo)/ags1/;P;D
'  <<IN
<time>20260664</time>
<tags>substancesummit ss</time>
<geo>asdsadsa</geo>
<time>20260664</time>
<tags>substancesummit ss</time>
<geo>asdsadsa</geo>
IN

sed -e '1{N;N' -e '};N;...;P;D'

sed -e ':next
    $!{/(.*n){3}/!{
        N;b next' -e '}
    };...cmds...;P;D'

cat input.txt | tr 'n' '@' | sed -e 's/txt@iam@interestedin/iaminterested@intxt/g' | tr '@' 'n' > output.txt