No — the role you pick at sign-up (Buyer, Supplier or Both) can't be switched by you inside the app.
Settings › My Account shows your role as read-only information; there is no control next to it to change it. The App language and Notifications preferences on that screen can be changed, but the role can't.
If your role needs to change, ask support. Use the Questions? Contact support link in the app and include your company name, your current role and the role you want to move to.
Don't confuse this with team roles. Owner, Manager and Member under Settings › Team Management control permissions inside your company, and the owner can adjust them any time with Change role. Buyer/Supplier is something else: it's how your account sits on the platform, independent of team permissions.
The hint on the sign-up screen that says you can change your role later from Settings has no screen behind it yet. So choose carefully up front: if you both buy and sell, pick Both from the start — that option unlocks both panels together.
Also asked as: How do I switch from buyer to supplier? · Can I change my account type later? · Where is role change in Settings? · I signed up with the wrong role, what now?