How average velocity is written
In most physics textbooks, average velocity is v̄, a v with a bar over it, read "v bar". Some books write v with the subscript "avg" instead, or use angle brackets: ⟨v⟩. It is defined as displacement divided by elapsed time: v̄ = Δx / Δt.
Velocity is a vector, so direction matters. If you walk 100 m east and back again in 200 s, your average velocity is 0 even though your average speed is 1 m/s. For motion with constant acceleration there is a shortcut: v̄ = (v₀ + v) / 2, the average of the starting and final velocities. The delta symbol Δ in these formulas means "change in".
What ṽ (v tilde) means
A tilde over the v is less common. In fluid mechanics, a tilde often marks a density-weighted (Favre) average, so ũ and ṽ can be averaged velocity components in turbulence equations. Elsewhere a tilde can mean an approximate or transformed value, and in statistics x̃ is the median.
If your teacher or textbook writes ṽ for average velocity, follow that convention. Otherwise v̄ is the safer choice, because most readers recognize the bar as "average", just like x̄ on the mean symbol page.
Combining marks and how to type them
ṽ exists as one ready-made character, U+1E7D (LATIN SMALL LETTER V WITH TILDE), so it displays reliably. v̄ does not: it is the letter v plus U+0304 COMBINING MACRON, and the vector v⃗ is v plus U+20D7 COMBINING RIGHT ARROW ABOVE. Combining marks are positioned by the font, so the bar or arrow can sit a little high, low or off center depending on the app.
In Word, type v, then 0304, and press Alt + X to add the bar (20D7 for the arrow). In a Word equation, v\bar followed by a space also works. In LaTeX, write \bar{v}, \tilde{v} or \vec{v}. Everywhere else, copy them from this page.