Usually, if you get Residence after an application that included a job or job offer, your Residence visa will be issued to include a Section 49(1) condition that you must work three months for that company (or a further three months, if you're already there). Then you can go back to INZ and show them financial proof that you have done the three months and are still employed by the firm, then (for no fee) they will issue another Residence visa that has no conditions on it. Once you have Residence with no conditions, you can do whatever you like that is legal. So at that point, you could change your job if you wanted to, making sure you keep to the terms of your contract about giving notice, etc..
If you have applied for Residence under SMC with a job offer, but you haven't got your visa yet because the application is still going through processing, and you then get a different job offer, you would have to make sure the second job offer is skilled, and also qualifies, and you would have to tell INZ so that they could check out the second job offer.