How to change a Schengen visa with single entry valid for only 15 days? [duplicate]

Can I extend my Schengen visa from Greece?

Mostly No. An extension is only granted under exceptional circumstances (such as serious illness) and requires the applicant to demonstrate availability of proof of funds. Source: Dutch government FAQ. For Greece a slightly out of date link is here. Quote from this link (emphasis mine)

Note that visa extensions are only granted under special extenuating circumstances, such as being in the hospital, having a serious car accident or a relative passing away. Visa extensions are not granted because of ignorance regarding visas and permits, careless planning, unexpectedly falling in love, taking illegal work and just because you want to travel more.

Assuming your circumstances are exceptional, the procedure is also given in the same link

Visitors who plan to stay longer than the expiration date of a Schengen or national visa while in Greece as a tourist, student or temporary business consultant should apply for a visa extension at the Alien’s Bureau Office or police station nearest their legal or temporary residence.

Apply between 7 to 25 days in advance of your visa’s expiration

For 15 days, you may not have enough time to even apply for an extension.

The other question:

What if I stay more than 15 days in my owns. Is not it that the Schengen visa is valid for 90 days?

I assume the Duration of Stay on your Visa sticker says 15 and also has a time window given in From and Until.

This means that you can enter Schengen area only ONCE and can stay only between the period mentioned in [From, Until] (both days included). However, the total length of your stay cannot exceed 15 days because that is the number of days authorised to you in Duration of Stay. The 90 days that you are referring to is the maximum duration permissible under Schengen short-stay visa.

If you stay for more than 15 days on your own, you will be breaking the rules of Visa issued to you by overstaying. Overstaying is a serious problem and can lead to many consequences such as fines upon exit, record in Visa Information System, problems in obtaining future visas and entry bans. In short, don't do it.

EDIT: For further reference, this French Government Link explains the schengen visa sticker and what each field signifies.

Speaking from experience - just trying to get something like that done in Greece would eat up (at least) all of your 15 days being bounced around offices :-(

The most useful advice I got from the police and border officials was:

• The cost of getting it done and the fine for overstaying is about the same. (was ~200 EU a decade ago)
• But the fine is a fixed amount however long you overstay, but extensions have to be renewed continuously.

The message was: Just overstay and pay the fine on the way out

The subtext was : Don't make me do the paperwork.

This is of course not good advice you should choose to follow, just an anecdote. But I spent months trying to do it the 'right' way, and failed, so followed the official advice (if not the official process).

If you are not planning to re-enter for a few years, it was also possible to just sit through a humiliating exit interview, get a red stamp on your passport and get on your plane. I've done that also, when unintentionally broke.