First of all, the implementation method is necessary and is part of the specification, so you must modify the specification.
Secondly, there will be no problem if the implementation method part has some overlap with the previous part, as long as Just explain the technical solution clearly
Third, if you are applying for a utility model, you can modify it within two months from the filing date; if it is an invention, you must file a request for substantive examination Within three months, or within three months of receiving the fact that the invention has entered the substantive examination procedure from the Patent Office, you can submit a revised claim. Modifications submitted outside these two time periods may not be accepted by the examiner.
Of course, no matter when the modification is submitted, it cannot go beyond the scope of the specification.