No, not as an alternative.
The B-complex is in addition to any other B12, injections or supplements, you are taking. It is simply a wise precaution to stop your other B's (B1, B2, B3, B5, B6 etc etc) from getting out of balance. You could take a very good well balanced multi-vitamin instead, it's really up to you. I was just putting forward my own experience, i.e. what I take.
The B12 deficiency itself must be treated with high dose B12. In the UK this is via hydroxocobalamin injections on the NHS, or methylcobalamin injections or infusions can be obtained privately. Many people source their own injectable B12 from mainland Europe where it is widely available over the counter, either cyanocobalamin or hydroxocobalamin. I am not advocating this, just pointing out that it happens.
Alternatives to injections are sublinguals, either tablets that melt under the tongue, mouth sprays or nasal sprays. These tend to be methylcobalamin, but you can get hydroxocobalamin sprays, and the most active form adenosylcobalamin (dibencozide).
The B12 pathway is complex and everyone is different. There's a tendency to say that Methyl is best, but actually lots of people do just fine on Cyano or Hydroxo. With the general lack of awareness surrounding B12 treatment options within the NHS it really is a case of trial and error to find out what works best for you.
I'm still on that journey myself. I get on very well with the Hydroxo injections, but adding in the Methyl sublinguals as well has been great. I used to think they weren't doing anything and stopped taking them for 3 days, I was wrong! At some point in the future I might add in some Adenosyl to see if that helps.
You really need to do your own research, and find what works best for you. I hope this helps.