How can I unlock iPhone if forgotten the passcode?

Your grandmother will need to use a computer to erase her phone following the directions in this link --> If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support

If she is not technically inclined, she may find it easier to visit her Apple Store Genius Bar. She WILL need to know her Apple ID and Apple ID Password, so make sure she know both. And in the future, encourage her to write important things like her passcode down somewhere safe for future recall.

If you forgot your iPhone passcode, use your Mac or PC to ...Apple SupportOfficial Apple Support › en-us

It may be easier to take the phone to an authorized Apple service shop in the area for help: Find Locations

As we previously noted above.......

It may be [will be] easier to take the phone to an authorized Apple service shop in the area for help: Find Locations

I’m sorry, there is no other solution. If she can not remember her passcode, the only option is to erase and restore the phone.

Then she will need to remember her password to access her phone and data. The serial number is irrelevant.

Please don't post personal information such as the serial number in this user to user forum.

We cannot help if your grandmother has no information about her phone. The serial number is of no value.

You have been provided with the correct instructions to reset the phone.

Someone who is familiar with the iPhone settings will need to visit your grandmother's home to help.