When extending your visa you need to consider whether you meet eligibility requirements. You need to ensure you have started the process before you visa expires to avoid ‘over staying’.
If you have a Tier 4 Visa, you have a sponsor and meet the eligibility requirements you may be able to extend your visa. You MUST be in the UK to apply apply for an extension to your visa. You should not apply more than 3 months before the start of your course. You may remain in the UK until you are given a decision.