Travelling alone, some tips please ?

I am 18 and I am travelling alone for the first time. I am from London and I'm travelling to Portugal for a week. I have some questions about travelling. Firstly, would it be possible to change my flights after I have booked it ? Because I have decided I want to go to Spain from Lisbon, Portugal. Secondly,... show more I am 18 and I am travelling alone for the first time. I am from London and I'm travelling to Portugal for a week.

I have some questions about travelling.

Firstly, would it be possible to change my flights after I have booked it ? Because I have decided I want to go to Spain from Lisbon, Portugal.

Secondly, would it be better for me to change the money from British pounds to the Portuguese Currency now or should I do it when I go to Portugal ?

Thirdly, if I want need extra cash can I just use my Debit Card abroad or would I need to let my bank know that I am travelling abroad to Portugal for a week and then possibly to Spain ?

Finally, we left Brexit so would I need a Visa and what about Vaccines ?

Thank you for reading my questions and answering to the best of your ability.
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