Applying for UK visa before first visa expires

Hello,

My mother- in- law has UK visa until 10th October. But we are planning for her to come to UK in August and stay until December.

1.What is the best way to apply for her visa.
2.Can we apply for visa before her current visa expires.
3.And if yes then when we should apply.
4.If UKBA grants new visa then do they grant visa after the first visa expires or from the date of application.

Thank you.

When was her last visit to the UK (exact dates of visit) and how long did she stay?

She came to Uk in October 2017 and returned back in December 2017.

She stayed for 2 and half months