1. Activate multi currency and automatic currency rates

2. Choose your rates provider ( Here example European Central Bank ).

3. Go on the European central bank website and export rates until the oldest date of your accounting entries

4.[Requires studio] Create the menu Rates linked to the model res.currency.rate

5. Import your rates on this new model. After that you can imagine that your database is up to date in terms of rates but unfortunately, it will change nothing retroactively.  

6. Here example with bills but you can imagine this with bank statement also for example the process is the same. First filter on those that are not in your main currency 

7. Reset those to draft or let those in draft in draft

8. Export them with external ids of entries & Lines & another info for example price

9. Reimport them (it will allow odoo to understand an update and so to recompute automatically rates). 

10. Repost those entries with finally the exact amount in your main currency on posted items