I own an GW Instek GDS-1102A oscilloscope. The firmware version instaled is 1.12, but the manual is for the 1.5 version. So, firmware instaled on my scope is very old
No, it is 7 minor versions newer than your manual.
and i would like to update it,
You would have to downgrade to have the same version as the manual.
but there is no firmware available for download on the Instek site.
This is normal for Instek. The Canadian site is an exception, but it is not well maintained. Maybe it is run by someone else (a distributor?).
All i have found is firmware v1.21 but for the GDS-1102 model.
Do not install GDS-1102 firmware on a GDS-1102A. They are different.
If you want GDS-1102A firmware you have to beg Instek to give it to you. Look for the after-sales service contacts on the Instek website. Be prepared to be pestered with a registration request and questions about your distributor and serial number. If they don't like the answers they might not send firmware.