install update

Excellent and thank you 👍

Again Terminal application and below command without " " Marks

" softwareupdate --fetch-full-installer --full-installer-version 12.5 "

This should download the Full Version of Monterey 12.5 ( About 12.2 GB ) and place a new Application in the applications Folder called " Install Monterey "

It will Not Auto Launch.

When ready and a TM Backup has been performed before the installation - just Launch it like another other Normal Application and follow the prompts.

If all goes well, upon the final reboot ( several will happen ) , Log in and check if 12.5 was installed.

If so, you can modify the previous Terminal command to read

" softwareupdate --fetch-full-installer --full-installer-version 12.6 "

The some procedure repeated and hopefully the 12.6 will be installed

You are welcome and thank you too 🇨🇦

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional questions may arise

An aside and in rebuttal to " Looks like some strange software on my machine makes me troubles. "

Understand, we are all working Remotely from all around the world. 

Therefore, we do not have the On-Hands experience the User ( you ) has with this computer.

The next best thing for us to having a semi - actual On-Hand experience  with this computer is to follow the steps below

Download the Application Etrecheck directly from the Developer. 

The application is free or paid from added features. 

It will take a Snap Shot -  both the hardware and software.

 The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Released Feb 10, 2022 Monterey macOS 12.2.1( 21D62 )

This may require performing an Intermediate Step Up to a newer version of Monterey before achieving the Monterey 12.6 installation.

Using the Terminal Application and the below command without Quotation Marks

"  softwareupdate --list-full-installers. "

What is the output of this command ?

It may list similar to below -->

softwareupdate --list-full-installers

Finding available software

Software Update found the following full installers:

* Title: macOS Monterey, Version: 12.6, Size: 12104042KiB, Build: 21G115

* Title: macOS Monterey, Version: 12.5.1, Size: 12114929KiB, Build: 21G83

* Title: macOS Monterey, Version: 12.5.1, Size: 12114929KiB, Build: 21G83

* Title: macOS Monterey, Version: 12.5, Size: 12112497KiB, Build: 21G72

* Title: macOS Monterey, Version: 12.4, Size: 11819688KiB, Build: 21F79

Welcome 🇨🇦

So the issue was solved ?

1 - Like poor Wifi connection

2 - Slow wifi connection

3 - Too many other people downloading at the same time and Apple Severs are overloaded

4 - Software on the computer interfering with the download process including AntiVirus Software

Notation: Disabling the AV Software may or may not solve the issue. Removal of the AV Software may increase the chances of the download to perform as expected.

5 - Running behind a VPN which may increase Packet losses or corruption of the download 

6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer. 

7 - Best Results are via Ethernet Cable to Router

Yes do keep us posted on the progress - time permitting 🇨🇦