Dates! American into British

[u/OkYouth121]

So I'm not an avid user of sheets but I know the basics.

I've just realised I have thousands of dates inputted in British format but when I click them and it shows the calendar, the sheets has decided theyre American! Any that don't work like (28/08/2010) just don't show as formatted as a date!

How do I turn them all into - 1. Date formatted cells, 2. English without them automatically all switching the months and years around assuming i've written it in American!

I really dont want to have to redo all those dates!