Changing Theme

Hello, I am so sorry if I double post, I just see that there is Theme section in this forum.. my apologies..

I’m in charge of running website for the NGO I’m working at, and this year we plan to renew it with a better look and appearance. Currently we are using Arras Theme and we’d like to change it into other (paid) theme. I am not very familiar with HTML and programming so I’m a bit worried, if I change the theme, all the posts will be there, is that correct? I would like to backup the files just in case, but we have more than 400 posts and I just want to make sure none of the posts, comments, and views are missing. Could anyone help me verify? So sorry for the troubles and thank you in advance!