Using the perl-based rename utility:

$ rename -n 's/^(d+)/sprintf "%02d", $1-47/e' *.mkv

rename(048. banana.mkv, 01. banana.mkv)

rename(049. apple.mkv, 02. apple.mkv)

rename(050. mango.mkv, 03. mango.mkv)

rename(051. pear.mkv, 04. pear.mkv)

rename(052. grape.mkv, 05. grape.mkv)

The trick here is the e modifier on the s/pattern/replacement/ substitution - which lets you evaluate the replacement string as a perl expression - including arithmetic $1-47 and padded formating via sprintf. Remove the -n when you are satisfied that it is doing the right thing.

If you only have a shell, then:

$ for f in *.mkv; do 

    n="${f%%.*}"

    printf -v newf '%02d.%s' "$((10#$n-47))" "${f#*.}"

    echo mv -- "$f" "$newf" 

  done

mv -- 048. banana.mkv 01. banana.mkv

mv -- 049. apple.mkv 02. apple.mkv

mv -- 050. mango.mkv 03. mango.mkv

mv -- 051. pear.mkv 04. pear.mkv

mv -- 052. grape.mkv 05. grape.mkv

Remove the echo once you are happy that it's doing the right thing.

With zsh and zmv

% autoload zmv

% setopt expanded_glob

%

% zmv -n '(*[0-9])(*.mkv)' '${(l:2::0:)$(( $1-47 ))}$2'

mv -- '048. banana.mkv' '01. banana.mkv'

mv -- '049. apple.mkv' '02. apple.mkv'

mv -- '050. mango.mkv' '03. mango.mkv'

mv -- '051. pear.mkv' '04. pear.mkv'

mv -- '052. grape.mkv' '05. grape.mkv'

In this case, the number formating is done using the l (letter ell) expansion modifier - from man zshexpn:

   l:expr::string1::string2:

          Pad the resulting words on the left.  Each word  will  be  trun‐

          cated if required and placed in a field expr characters wide.

As with rename, remove the -n to actually change the names.