Changing employer within UK

I have a Tier 2 (General) visa with Employer A and have been in the job for 3 years, but I've received a job offer and a CoS from Employer B. From what I understand, I need to apply for a new visa. But when I try to apply online, I get a message saying 'You are unable to apply for a UK visa from within the UK.'

Could someone advise on the process? Thanks!

Is it a resctricted or unrestricted COS that was issued by the Employer B?

It's an unrestricted CoS. Employer B was advised by the Home Office that that's what was required, since I possess a residence permit and currently live in the UK.

Just an update for anyone else with this same question.

I called the Immigration office and asked which application link I should use on the website (Apply outside the UK, Extend, or Switch). It turns out that even though the website states very clearly several times that you're not supposed to use the 'Extend your visa' option if you're changing employers...that's indeed the link you're supposed to click. This will take you to the right tool.