How can I update the firmware if provider installed a Custom?

I bought the phone from my provider. it supposed to be a dual sim, however the provider has installed a custom built version/software blocking me from updating to android versions as well as using other sims.

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crwdns2934109:0crwdne2934109:0

crwdns2889612:0crwdne2889612:0 0

crwdns2942205:01crwdne2942205:0:

Make sure before you try anything that updating it won't void something with your business contract, most of the time if there is a custom OS there is a reason for it...

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