Personally I would just go for it, but with an important detail...
Don't just put a note in saying "The price will now be £14, Thanks, WINDOW CLEANER"
With such a large increase, you really have to give them more information to get them on-side. Give people a reasonable explanation and they will support you.
In my experience, people are willing to pay way more than is generally thought. I have wacked prices up because I hated doing the job and they still happily paid. (EG price rise from £12 to £24).
Once I made a typo when texting a customer with a price, I put £29 instead of £19. I only realised my mistake when the payment came from her for £29.
I quoted one house on the end of a row for £40, the customer said "yes, that's fine". When I came to actually do the house, I had made a mistake, I had included next door as well (I thought it was all one house) but I continued to do that small house for £40 for years until I moved out of the area.
Was this a rip-off? NO! I offered to do the job at a price, and the customer accepted. I then went on to perform the service to a high standard absolutely regularly as promised. No-one was ripped off.
So the moral of the story is... charge a good price. People are willing to pay for good service.
An underpriced customer is no use to you. It is simply taking up time that could be spent doing a well priced customer.