Please I have a 6 months multiple entry Visa to the UK which expires on the 17th of June 2019. I have entered and returned back to my country on the 15th January 2019. I have cause to return for the same purpose on the 30th of June 2019. Please how can I go about the extension; can I enter before 17th June and seek for the extension or apply for the extension before entry from my country and what are the chances of getting the extension without overstaying my visa as a default?
You can apply to extend your stay however you can only do it from inside the UK and it rarely makes sense except in a true emergency or unexpected situation, not a foreseen situation like yours. The cost, increased probablility of inadvertently overstaying, long processing time, etc all combine to make it a poor choice.
In your case the best option is to apply for a new visa before entering the UK. Note that the immigration officer at the airport can turn you away if they realize your plan.
If you still want to continue down that path, see below:
You may be able to extend your visa as long as the total time you spend in the UK is less than 6 months. For example if you apply for a 3 month visa, you can apply to extend it for 3 more months.
Read the guidance to find out if you can extend your visa.
You should apply before your current visa expires.
A visitor (standard) and a visitor for marriage or civil partnership, who was granted a visit visa or leave to enter for less than 6 months may be granted an extension of stay as a visitor so that the total period they can remain in the UK (including both the original grant and the extension of stay) does not exceed 6 months.